I tracked it down to this commit, but after reading it I still don't understand what does this do other than drawing the button in different colors after clicking.

https://github.com/nesbox/TIC-80/commit/6a19f59b9d00a09b3318e3270194b162c6ca34b0

Does anyone know what does it do?  Or is this a WIP feature?

If the pin is black, then the bank selection buttons (0...7) will affect all sections of the cart. If gray, then it'll only affect the current section (sprites/map/SFX/music). Also, please don't use code banks, as this feature was removed in 0.80.0-dev due to bugs.