It's not true for a place-and-route engine, so why does it have to be true for a C compiler?

Nobody else cares. If you do, that's great, I guess... but you'll be outcompeted by people who don't.

I'm glad you asked! https://reproducible-builds.org/

That's an advertisement, not an answer.

Did you really read and understand this page in the 1 minute between my post and your reply or did you write a dismissive answer immediately?

Eh, I'll get an LLM to give me a summary later.

In the meantime: no, deterministic code generation isn't necessary, and anyone who says it is is wrong.