I was a member of a German club on Duolingo and now it seems to not exist?
You were (probably) kicked out.
Clubs can only host up to 15 members, so admins would want their clubs to be populated by active members only. If you were unactive, you were probably kicked out. Another thing is that admins can set their own standards for new members, so for example the club I'm in says "only 100+ day streaks" and it's called "streakers" so anyone with less that that is soon to be kicked out. The last common cause is cheating; if you test out of too many skills, youll be suspected of either already knowing the language, or hacking, so you'll be kicked out. It's all pretty much up to the admins and your pattern of participation.