I can't claim to be a 100% sure to be honest, but if the information on their privacy info page is accurate, it shouldn't ask for any android system permissions at all, including internet access:
In particular, MessagEase does not seek Internet access permission.
I've used MessagEase for many years now and I absolutely love it. It's a very different kind of keyboard and so takes a while to get used to but once you've learned to use it properly you can type a lot faster with it on a touch screen with limited real estate.
Timeless classic.
Nice! I hope you will like it.