One way to think of this is that the players and the GM are all trying to tell a story together, and dice rolls exist to resolve conflicts between the stories they're trying to tell. Or if you prefer, conflicts between their stories and a world that has other ideas.

Normally the player wants something to happen, and the GM calls for a a die roll, the GM is represents the world opposing that event... and that's one of the many roles they fulfill at the table. However if the GM and the players all agree that the story should go the same way, you don't need to roll a die at all. That means if the player thinks they made a persuasive argument, and the GM believes the NPC should be convinced by it, then the GM doesn't have to say "roll persuasion" they can just say "yes that works"

Perhaps a better example - you don't always need to make a player roll to find traps when they're looking, especially if their score is much higher than the DC - you can just say "while investigating, you find this trap". Maybe your story is more interesting because the trap is ingenious and needs something clever to disarm it, maybe it can't be disarmed, and triggering it is a choice they have to make or go another way. Maybe the existence of the trap is only there to provide context or detail to the group, and it's not intended to be a threat.

This goes for attacks too. Almost all of the time, the players will have less fun if they know the world is pulling its punches, because they'll know there's no risk and they'll always win - it's not fun or satisfying to beat a challenge that was rigged in your favour after all.

But... if the GM knows for sure that everyone will be miserable if (x character) dies, and they think it will make the game or the story worse, they can just roll a die behind the screen and not look at it, then say "oh it missed" just... don't do it every time.

A common way to get around explicitly giving the HP of a monster and telling them nothing is the "They look... " rule. When they ask how many HP the baddie has left, tell them "They look injured, but not enough to hinder them" or "they look bloody and totally messed up" etc. As a rule of thumb, you can decide their health into quarters and come up with a common phrase for each, or come up with them on the fly depending on the situation: "Grog's hammer has left some of its ribs broken, but it looks healthy enough to keep fighting for a while."

4e had a specific status called "bloodied" that creatures gained when they dropped below half HP, this represented that one of the attacks on them has been a telling enough blow that they're showing signs of injury. I brought this with me to 5e, because it's a useful contextualizer for players to get a feel for how well they're doing.

One advantage of this system (especially for new DMs) is that if you massively overspec an encounter and the players are in trouble, you have some time to realize it's going badly, and can drop the monster's HP pool a little to compensate.

One advantage of this system (especially for experienced groups) is that if the party are doing badly, and haven't realized it - the moment you say "right, the enemy is bloodied" they realize that they've "only" done half the dragon's HP, and are reminded that retreat is an option they can take. Remember that if the whole party decides to retreat, it can be good to drop out of combat, and make the attempted retreat a skill-based challenge, rather than trying to run the retreat on the combat grid. 5e makes it very very difficult for creatures to "outrun" other creatures that are trying to kill them, and the combat system doesn't handle retreating well.

If you want a mechanic for it, ask the player who wants to know to make a medicine check - this can add value to the medicine skill (which doesn't see a lot of play):

If they beat 10, you give them a very rough idea, like "they've been hit a couple of times but they look like they're going strong"

If they beat 15, give them a loose fraction to the closest 1/4 or so "they've lost about 1/4 of their HP" etc

If they beat 20, give them a number to the nearest 5 or 10 (depending on if you're low or high level.)

Increase these DCs by 5 if the monster is something that they'd be unfamiliar with the biology of - how easy is it to tell how hurt an air elemental is? not very.

An important thing to always remember is, every table is different, if one thing works for your group - do that, don't think that you have to follow any piece of advice just because it came from someone who sounded authoritative, or gave you a lot of numbers.

That's a player guide lol. Only the DM should fudge.

Played a control/support wizard for almost two years. Died to a power word kill and BBG used his soul as a bargaining chip. Party was too full of themselves and newer players, they called his bluff, my wizard was perma dead. The rest of the session was them as players and characters coming to terms with his death. It was god damn beautiful and one of my favorite memories in gaming.

Please DMs, kill your ~~players~~ player's characters. For the character development.

Edit: being neurodivergent I sometimes forget that people can have personal feelings that I find illogical, so as the comment under mine says; please make sure your player or players are not going to be traumatized if you kill their characters. As a DM I have always done this, because even if they are killed off I want the players input on how it goes, but that is for narrative reasons and I had not considered how badly it could have gone if I hadn't been asking. I have never been asked by a DM, it just doesn't bother me because to me it is a part of the fun and magic of TTRPGs.

Also, there totally isnt like 5 different ways for the players to rez a pc.

Depends on character level, setting, game tone. Not a universal solution to a universal problem.

And lets forget about habing NPCs do the rezing as a sidequest.

Not every game lends itself well to an unexpected sidequest. Also what is the dead PC's owner to do in the interim? This introduces a lot of questions and is also not a universal solution.

Did you read how defeat works in Fate? You can have death.

https://fate-srd.com/fate-core/conceding-conflict . If you don't want to go look it up, I'll summarize here:

In a conflict, before a roll is made, you can Concede. This is a Player action, not a character action. It means that you give up the conflict, but you get a say in what happens. You don't get whatever you were fighting over, but so long as the group agrees it's reasonable you can get something like "taken prisoner" or "left for dead." You also get a Fate point, which is nice. (D&D also has an extremely lackluster meta currency system, but that's a separate discussion). Note that it's not the DM just deciding what happens to you. That's for getting Taken Out.

If you instead let the roll happen, and you take more stress (damage) than you can hold, you instead get Taken Out. When that happens, you have no say. Barring normal social contract stuff, whoever was coming at you has free rein to just be like "And the spell explodes your head."

This is in the rules. To me that's much better than D&D's wishy-washy "maybe the DM will do this or that" standard. I don't want to hash this out at every single table I join from first principles.

D&D kind of sucks because it leaves a lot of important things up to the DM, so you get wildly different experiences depending on whatever half-baked whims this table has. And you have to have these conversations over and over again. And some people never will know there's other ways things could be, and leave the hobby or just be unhappy.

Some people might say "leaving more up to the DM is better" but that's wrong. Clearly going maximum calvinball "whatever the DM says in this moment" is not the platonic ideal of a game. At least not for me or anyone I know. Some rules are important. D&D is missing some important ones. And has too many rules in other places.

BURN THE HERETIC!