OpenBoard is my favorite.
There is the OpenBoard fork:
https://github.com/Helium314/openboard
FlorisBoard hasn't had official updates in ages but beta builds are still worked on:
https://github.com/florisboard/florisboard/releases
I know there are others too. Some also use a real keyboard app and disable Network access.
Discussion
OpenBoard fork will rename to HeliumBoard or something soon to differentiate between the two. The FlorisBoard UI is better imo but OpenBoard is more functional.