No such thing happened.

You're likely referring to this paper quoted in Wikipedia: https://eprint.iacr.org/2016/374.pdf

This is 1. not about SHA256, but about the truncated sha2 algorithms and, more important, 2. it's an attack on reduced-round versions of these algorithms. That's a common thing in cryptoanalysis to do. You're basically saying "we have no way of attacking the full algorithm, so let's build a version which is much less secure and try to attack that". That's a valid thing to do for research purposes, but it needs to be interpreted correctly. "I can break this massively weaker version of the algorithm" is very different from "I have shown a practical attack on the real algorithm".

> You're proposing to add extra complexity for some hypothetical scenario that is unlikely to happen

I disagree. It's evidence that the given path is not fruitful for breaking SHA256 in the future. When numerous researchers worldwide have attacked a function, and only broken X out of N rounds, for X << N, and future research hasn't been able to improve X for years, that's pretty good evidence that the technique used isn't going to continue to apply for more rounds. The existence of a multi-billion dollar bug bounty for breaking SHA256 (Bitcoin) that's gone unclaimed for years is further evidence that it's quite strong.