Hackers Keyboard has that via ctrl+z, but only in horizontal mode. For this only i had my Galaxy S3 in horizontal per default. But can't do that with modern phones' aspect ratio and i'm missing undo since then.
So, uh, after writing that, I realized the most recent FlorisBoard update actually removed quite a few features and this might not be true anymore. 😅
For example, FlorisBoard had glide typing built-in, now it doesn't anymore. Apparently, the dev is working on re-implementing it along with word suggestions (which are currently missing, too).
I actually happen to not need these features, so I kind of forgot. And this most recent update came with an add-on store integration, which seems like it could become quite cool in the future, when there's more add-ons: https://beta.addons.florisboard.org/
Aside from that, I think, FlorisBoard still has a few more configuration options, but on the other hand, HeliBoard has a Kaomoji tab in its emoji selector, so I guess, I'll give HeliBoard another shot. ¯\_(ツ)_/¯