After more than two years FlorisBoard (an open-source keyboard which respects your privacy) 0.4.0 is finally released with many new features including MDY support, addons store, incognito mode, major smartbar rework and many other bugfixes and improvements.
Regarding the spell checking (from GitHub release notes):
Word suggestions/spell checking are not included in the current releases and are a major goal for the v0.5 milestone.
Still no word suggestions?? Wow.
Anyway, Heliboard exists.
It sounds like at least some of the features released in 4.0 were needed to make that work. Glide typing got disabled as a result of the new framework, so it’s definitely on its way (more now than before)