There is the ability for sliders to be disabled, just like any other button. However, even though the font is grayed out, the marker, which shows where the value is located, is not when hovering over it. So it indicated that it is still movable, which is not true.
This bug is most likely caused due to it just not happening in the vanilla game and it not needing to be implemented at this point in time.
Disabled FOV slider
Reproducing the issue
At the time of writing this, there are no disabled sliders (at least to my knowledge) in the game. So, in order to reproduce this, modify the source game or create an external mod for it.