That's no longer true: https://www.intel.com/content/www/us/en/developer/articles/t...
x86 hardware does not guarantee anything unless you enable "Data Operand Independent Timing Mode", which is disabled by default and AFAIK can only be enabled by kernel-level code. So unless operating systems grow new syscalls to enable this mode, you're just out of luck on modern hardware.
In the most purely theoretical sense, you are correct, but everything Intel and AMD has said indicates they will still offer strong guarantees on the DOIT instructions:
In other words, they have announced the possibility of breaking that guarantee years before touching it, which is something the clang developers would never do.
