FlorisBoard is in f-droid, and has swipe. What it doesn't currently have, and has been in the "to do" list for - I don't know, over a year - is autocorrect. This makes swipe far less effective than on other boards. But it does work well; there are just a lot of spelling errors to fix.
If it's feature complete, a keyboard doesn't need regular updates. According to the other thread it's not feature complete, so it could be abandoned. Since it's FOSS any enthusiastic person could stop it from being abandoned.
Oh, yes. Absolutely. The "this software is newer therefore better" or "it gets more updates so I know it's better software" affirmations make about as much sense as "my complete rewrite of Shakespeare's work using a vocabulary of 200 words is better because it is newer" or "this mathematical proof uses more paper, so I know it's better".
This "argumentum ad novitatem" fallacy grinds my gear almost as much as the "argumentum ad populum" one...
Unfortunately, both argumentations are really common in FOSS (and in software in general). π
It freezes. For minutes at a time. Not often, but minutes are too long, and you cannot kill it. Also, it auto hides in the middle of typing when you type "too fast", there are words you simply cannot swipe ("never" being one, it always gives "nerve"), some that constantly are guessed wrong (common words replaced by a rather obscure alternative), and sometimes, when you type "too fast", the cursor moves one or to positions backwards as you type.
Honestly I appreciate the effort it must have been to code, but I'm sad it has all those issues. That being said, it is still more usable than Florisboard...
The best one I found so far was the AOSP keyboard with swiping enabled via a proprietary blob. But, well, it's not entirely opensource...