Yes, that is too old for a new phone considering it's already past its end-of-life for both official support and your OS. I'm not sure why you'd recommend them to buy new either - a phone like that is only going to be good value if you pick up a used one for cheap. A new model will be massively overpriced for what it is (and may not even be new, just refurbished and repackaged).

I think it's a bit too old, if you want to stay in the pixel ecosystem maybe try to grab a 6, 6a or 6 pro. They are around $250, and they are great!

The one we can't talk about...

I don't get it ? Why can't we say it's name ?

4a is end of life already, so no firmware updates from Google. GrapheneOS has legacy builds available for it but doesn't recommend using them, and they might go away anytime soon

get a used device which is still properly supported, don't buy brand new e-waste

What's the OS we can't talk about?

The Google Pixel 4a is officially end-of-life and doesn't get any software and security updates anymore (https://endoflife.date/pixel).

Can someone explain to me under what circumstances would using an old phone be risky (under a common reasonable threat model)?

I'm using a 4a right now which I bought last year, refurbished. It's a great phone and has a headphone jack. If you're concerend about updates, install an alternative OS. If you want to degoogle that should be the path anyway.

I bought a used Pixel 5 in Feb for my daily driver. Replaced my Pixel 3 only because the power button was flaky. They both still run great. By my standards, getting two years out of a phone I paid $150 for is better than getting three years out of a $700 phone.

You can install LineageOS or e/OS on it (instead of Graphene, if that's too controversial), and then the 4a is a good phone to use.

Yes, it's too old. Does not receive software updates anymore. The newer a-series of phones are still quite a bit larger than the 4a but also quite a bit smaller than the 8 or especially 8 "Pro" or whatever the fuck stupid name they're giving phones these days.