As a heavy user of the “Disable Masters” custom parameter, I think it would make more sense to also have a “Keep Masters” option. Let me explain why.
Imagine I have a font with a few variable font instances that use the “Disable Masters” parameter. Later on, I update the .glyphs file by adding new masters—let’s say a new width. In that case, I have to go back and manually update all the instances to include or exclude these new masters.
But if there were a “Keep Masters” parameter, the behavior would be more predictable: only the explicitly listed masters would be kept, and any newly added ones would automatically be ignored—without needing to touch each instance again. This would make working with evolving designspaces much more efficient 