You can still upload addons for Thunderbird, FF *-56 and ESR. The problem is the system doesn’t allow multiple systems (WE & legacy) under the same ID.
The option is to create a new entry for one of them (WE version or the legacy version). It will be inconvenient since there is no automatic method of transferring users to the new addon.
That is all I can think of. Maybe @jorgev has a better suggestion.