I received the same answer from Jottacloud after asking them what and how they will suppoert in the future:
We are looking into the issue Duplicati users are reporting regarding authentication, to see if we can make a temporary fix as a one-off for this specific issue. No change in our policy regarding support of third-party apps/services.
The question - whether the end of service was announced (prior the switch off) in a user friendly manner, in order the affected customer can find alternativ solution - wasn’t answered yet.