The “Enforce Compatibility Check” between the behavior of Glyphs 2 and 3. In G2, it would only complain if there is an instance between two masters. Otherwise, e.g. if you have instance only at the same places as your masters, it would not complain. Glyphs 3 always checks all masters by default because it assumes you intend to export as a variable font.
that’s a pity, as sometimes when we are making things we love we might decide not to export a variable font, and prefer to not be distracted by red bars. why not remove the option to unenforce compatibility check?
Yes. There is a custom parameter called Enforce Compatibility Check. Add it to Font Info > Font, turn the left checkbox on (activates the parameter), turn the right checkbox off (tells it to not check for compatibility).