I always thought people like Kang basically just got plastic surgery once they had sufficient influence to be able to afford it.

My other thought is that most of the components of ridges are a dominant genetic trait (kind of like how the Klingon ridges were prominent in Romulan-Klingon hybrids or B'Elonna), and that through a gradual process the virus-affected Klingons interbred with unaffected Klingons until ridges returned.

As for DIS Klingons, I have several thoughts. Since the Klingons go back to normal by SNW, it could be possible that they just decided to retcon the DIS style away all together. Another weird thought is that they could be some sort of genetic glitch encountered with augment-normal hybrid Klingons where they got the ridges but their melanin's all wonky, resulting baldness and either albinism or an "anti-albinism". Once again, the interbreeding meant that eventually, these traits disappeared.

To explain why we tend to see the same type of Klingon together, there could be social biases to keep oneself surrounded by the same type of Klingon; this maybe slows interbreeding enough that communities of several types of Klingons still existed by the 23rd century, but had largely merged back into the normal-ish Klingon by the 24th.

The difference between the klingons in TOS and the TOS motion pictures, was that klingons always looked like that, and in DS9, according to Worf, it was due to some unspecified thing that is not discussed with non-Klingons.

But other than that, not really. The official position likely has not changed.

In fandom, it varies. Some people treat it as a cosmetic choice, and that Klingons underwent changes as necessary, and others might stick to the one interpretation of what klingons look like for all of them.

Personally, I'm of the Diaspora opinion, where the varieties of Klingon all coexist, where some of the changes are racial, like they are in humans, and others not.

For example, T'Kuvma house were Klingon supremacists, so it seems likely that some of their more exaggerated features were due to genetic modification on their part to try and enhance their Klingon attributes and remain Klingon.

But B'Elanna? She's just like that.