It is not a problem that different CPUs have different execution time, the problem is if the same CPU, running the same instruction has a timing difference depending on the data it operates on. In this regard CPUs have actually gotten better, specifically because it is a feature that AMD and Intel has pursued.

Not always. At least for RISC-V there is the Zkt extension which guarantees data independent execution time for some instructions. I assume there's something similar for ARM and x86.

It does pretty much require you to write assembly though. I think it would definitely make sense to have some kind of `[constant_time]` attribute for C++ that instructed the compiler to ensure the code is constant time.

If you want to get very paranoid most instructions probably use slightly different amounts of power for different operands which will change thermal output which will affect CPU throttling. I'm not sure there are any true constant time instructions on modern high-performance CPUs. I think we have just agreed that some instructions are as close as we can reasonably get to constant time.

Or microcode updates to existing CPUs!