It's ethereum's difficulty bomb but for bitcoin! Introduced specially for knots users in 2017, without even a PR, let alone any review.
It is the -softwareexpiry startup option, which is default on. I think most people do not know about it.
https://github.com/bitcoinknots/bitcoin/blob/28.x-knots/src/validation.cpp#L4458
Discussion
Should just do vulnerability disclosures on v(N-2) to keep people upgrading 😂
Eh, there's always the option of backporting fixes to of versions yourself if you really want, or, I guess, paying someone to do that. Or just run the old software and mitigate the risks in some other way - just not being a listening node is often pretty good.