Is it a global option set?
Did you publish all customizations, not just the single entity?
You are not alone.
We have struck this before on the odd occasion.
There seemed to be no logic.
For us the field had been removed from everywhere it was telling us to remove it from, and was no-longer actually referenced by anything we could configure/code, but the dependencies persisted.
We made the field non-searchable (so people didn’t see it in Advanced Find lists), renamed it as (do not use) and left it in the solution.
Not ideal, but generally not the end of the world.