Analysis from Kévìn: Solid has a disabled WIP replacement for this. API looks good and complete, and initial tests show this to work. So it might actually be an option to move to this.
However it turns out that the old implementation is actually much closer to the rest of Solid than the new one. It might therefore be more consistent to just promote the old one to Solid.

Input from @broulik and @davidedmundson: Split in two parts: one part is already reimplemented in plasma workspace library and only relevant for the shell. What remains is the screen lock inhibition API that should be added to Solid.

This would suggest that at least the new (disabled) job-based API in Solid is no longer needed and can go away.

Given this is new API that we can add any time I'm moving this to low priority