Just to chime in, for well over a week now I cannot update apps on my Mac, 2 iPhones (iOS 13.2.3 and iOS 12.4.3), and one iPad Pro that are all using the same AppleId account. Being a developer, yes TestFlight has been used on these devices however:
I also have other devices (iOS 13.2.3) that TestFlight was also used on and they can still update just fine. These other devices are using other, non-developer account linked AppleIds.
So I don't think it is as simple as whether TestFlight was used or not, or even whether sandbox purchases where made, etc... as I see that some AppleIds work and some don't regardless of whether TestFlight apps (or even direct developer builds over USB) where installed.
I also have other devices (iOS 13.2.3) that TestFlight was also used on and they can still update just fine. These other devices are using other, non-developer account linked AppleIds.
So I don't think it is as simple as whether TestFlight was used or not, or even whether sandbox purchases where made, etc... as I see that some AppleIds work and some don't regardless of whether TestFlight apps (or even direct developer builds over USB) where installed.