Not technically a fault of Solus, I hope...
I had a fully updated old Solus install I used daily, then wiped and installed a fresh 4.3 expecting nothing to change. But now my thunderbolt USB-C external screen doesn't work. I can't remember if I was on LTS or Current before, and now of course I'm on Current with the new install. I see in old threads that 5.13 caused a regression for thunderbolt, but was soon fixed. Does anyone have experience or knowledge about this situation?
If I 1) boot with the device plugged in 2) then boltctl monitor it shows some warnings about uid and device name, but this is probably unimportant if it's a kernel issue.
I've disabled Thunderbolt security in BIOS just to be extra encouraging.
LTS is all the way back on 4.x but I'll give it a shot.
I have tried both 5.13 and 5.14 available to me:

  com.solus-project.current.5.14.21-210
* com.solus-project.current.5.13.1-187

[EDIT]
Can't even boot with the 4.x kernel. Will try compiling 5.12...

Arg, building the 5.12 kernel gets stuck when answering an automated question
KCSAN: dynamic data race detector (KCSAN) [N/y/?] (NEW) Or "Please Press enter to hang forever."

@kaktuspalme Thanks for the help. Fortunately, all that kind of stuff was taken care of when I did the git checkout linux-current-5.12.13-185 to make sure I get exactly everything needed to rebuild the old kernel package. I've no idea why this gets stuck.

    grumbly Maybe the new option is only available with a new gcc version or something. But anyway. Add the CONFIG_KSCAN=n to files/config (or =y in case you want it)
    Probably you should be able to build it then.

      @kaktuspalme Thanks! That wins the battle. Unfortunately, my war rages on!
      Solus is refusing to work with my external display now -- and get this -- it doesn't work now in live boots of 4.0, 4.1, 4.2 or (current) 4.3. But it used to work for my prior updated 4.x installation just last week. Just to make sure something wasn't damaged, I booted up Ubuntu live from USB and Bam! the external display works upon plugging it in. I'll consider this case closed with @kaktuspalme 's answer and open a new post wondering how to get the display to work.

      In thinking about this more, I realized Ubuntu "comes with everything" activated, and I hadn't activated the NVidia drivers in Solus. This was the solution. I guess I thought the basic drivers would be capable of this with the Intel integrated graphics. Nope!