Even if you didn't use a HDD, that option for HDD support was useful for performance tweaks.

For next big update+PL, CDPR wrote they'll "discontinue active support".

Does that mean this option and the like will vanish?
Will they be gone from UI options only, or also from the config files?

It basically means that they'll no longer be testing on systems with mechanical hard drives - SSDs only - so they'll no longer be able to see how using one affects performance or stability. So, if you have to install any part of a game on one and start having problems, tech support can just say "sorry, we don't support those kinds of setup anymore (regardless of whether or not it may be the cause of your problem)".

This is the reason why it's only being released on next gen consoles as well, it must be a solid state hard drive it must have modern graphic system. Can't make it work on older stuff. The updates and enhancements are too great it just doesn't work on old hardware. A HDD is considered antiquated hardware now along with any graphics card that doesn't support RTX hence the 1060 being the bottom of the barrel card supported.