Not by default usually but yes, you need to do a lot of work to set all necessary configurations and sometimes provide your own RNG seed for things which insist on random looking values.

Yeah, but it does take a lot of work to coerce them to do it.

GCC is honestly obnoxious, and you have to do a bunch of unintuitive things to get it. The compile stage needs the built-in RNG seeded, parent file paths stripped, you need to ensure that the date/time macros are not used anywhere, and you need to ensure that all command line flags are passed in the exact same order every time.

I just went through this with GCC16 on a new project.

Not only can they but certified toolchains exist in the safety critical space. Medical, aerospace, nuclear projects, and the like are often required to use and procure such toolchains as part of their validation.