It was meant for testing one of their UI redesigns.
sh.reddit.com
seems to still work
Android roadmap: https://developer.thunderbird.net/planning/android-roadmap
There's fork that gets updates
Voyager
https://filterlists.com/lists/actually-legitimate-url-shortener-tool is bigger list and is updated more often.
Not available on Android.
From README:
The current focus is on implementing next-word suggestions, which has become complicated/hit a snag and is taking more time than originally planned. Updates will come when this issue is resolved. Thank you for being patient.
It’s still being developed:
https://github.com/florisboard/florisboard/releases