[srdjanr]

It definitely is visible, OS login screens usually warn you if caps lock is on when typing password

[bnjms]

Thanks for the correction. I understand it is seen but wonder if it was in keyboard only in the past.

[ptx]

I don't think that would work, as capslock should only affect the letter keys (not e.g. turn "1" into "!") and the keyboard doesn't know which those keys are, as it depends on the layout configured in the OS.

[brewmarche]

Also, some layouts use shift lock (so actually turning 1 into !) instead of caps lock