Flat out I’ll say Thanks to Xojo for the 2.1 decision to revert the event name changes. Of all the changes that one was probably the hardest one to deal with as there’s no easy way to #if around an event definition or event handler.
Properties may still cause some grief if you start a project in 2019r2 or r2.1 since they don’t retain the “old” values” so going back to an old version can be tricky if you start in 2019r2 or 2.1. At least this can be worked around by starting projects in 2019r1.1 or older and its fairly simple to make it so every project behaves like this by using a template that overrides the normal built in one.
if you use a template in R2+ one still needs to do a Save As… Which then resets teh min IDE version, no?