Must say it's a little bit lame that they are boycotting iOS. I will not change phone just to use this app but it would be nice to be able to replace SwiftKey with this.
It's notoriously not on f-droid. The way FUTO licenses and/or builds its thing is made deliberately incompatible with f-droid's main repository.
You can add the futo repository to the f-droid client, but when people talk about f-droid they really mean the main repository, not the extra hoops to add less trusted third parties.