If, as a developer, your app isn’t ready for iOS 14, it is your own fault. The beta is going on for a while for a reason. You do have access to the beta code and you should have been testing it on the newer environment. It never is supposed to be a night and day change shift should you had done your homework. Bugs might show up if you use obscure or undocumented features, but anything not falling onto such categories, or a tremendous bad luck, it is again you being lazy.
I had all my apps ready in between iOS 14 Betas 2 and 3, and afterwards it was just a matter to recompile them and run my compatibility, validation and regression tests. It was iOS14 GM ready before I went to bed last night and it is already online for users to download.
Don’t blame Apple for what you could had done yourself.