[lifthrasiir]

You haven't changed but compilers have changed, unfortunately. Unless you stick on -O0 or -fno-strict-aliasing all the time, the chance is that your UB-ridden code can break in the future with more powerful compilers exploiting more UBs. So that's why you have to care now if you didn't so far. (Or you can argue that optimizations should be turned off, which is indeed another valid, though uncommon, answer preferred by djb for example.)

[tyfighter]

Actually, I have changed, and I've changed corporate C/C++ MANY times to satisfy compiler upgrades. It is always just shitty code. It's never something insidious. Bugs happen. It probably wasn't your intention, but you've made UB sound pretty awesome.

[lifthrasiir]

If you are mainly dealing with shitty code, which frequently contains an UB, then most UBs you've encountered should have been from shitty code. That doesn't mean all UBs indicate shitty code.