I’ve been having a look for one but can’t find any. Am I missing something obvious?
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.
The problem is that It looks abandoned. Not a new release in a year
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.
expired
AnySoftKeyboard, I use it and love it, but it need a lot of time to be configured exactly like you want
I looked at it briefly and just want to confirm - it doesn’t support simultaneous use of multiple languages, od does it?
It does! You can download the additional languages and then configure it how you want (a very common one is to swipe on the space bar)