I had exactly the same experience, at about the same time. Had been hearing good things about Plex so decided to try it out. Immediately noped out when it required me to create an account with them. Similar to you I looked around and found it to be a relatively new change.
Frankly baffling to me that anyone with the wherewithal to self-host was okay with it.
95% as soon as I unplugged it
This is normal for the Deck and is done to help extend battery life. Keeping a battery at 100% charge for extended periods accelerates its degredation. The Deck will charge to 100%, and you should see this if you immediately take it off the charger as soon as it finishes charging, but then lets it drain down to about 95% and maintain there if you just leave it plugged in.
Jellyfin has some security issues that, depending on who you ask, are either critical vulnerabilities that make it completely unsafe to expose to the Internet or largely unconcerning for regular users.