I did a quick review of what FOSS options are currently out there
https://news.ycombinator.com/item?id=47936103
There are a plethora of open-source implementations available on F-Droid. They need to be looked at for privacy before choosing one, but there are completely offline ones.
alt Hacker News
There are a plethora of open-source implementations available on F-Droid. They need to be looked at for privacy before choosing one, but there are completely offline ones.