I saw a number of posts related to locking custom fields, but it seemed that these were related to editing the custom field properties, as opposed to locking the ability to change a selection.
I have a number of custom fields that automatically update throughout the task’s progress through our pipeline. For the purposes of our users, these are effectively “read-only”, though that does not prevent users from manually changing values. Having the ability to lock specific custom fields from being changed by anyone other than Project Admins would be incredibly helpful in preventing unwanted actions in our pipeline.
The problem in our case is that there are some Rules in place that trigger on the change of custom field values, typically triggered by previous rule. So tasks end up in all sorts when custom fields are manually changed, and triggered by rules it wasn’t intended to be triggered by.