Not sure what you mean by modal keyboard, but there is fully customizable row of keys in termux.
I disabled it, because I use special android keyboard app[0] with a custom layout made from scratch[1]: all common symbols, arrows, esc, ctrl, alt, forward delete, paste and enter gestures etc. etc.
[0] - https://jbak2.ucoz.net/ (yes it's horrible website, but it's free and easy to maintain for the handicapped developer)
I disabled it, because I use special android keyboard app[0] with a custom layout made from scratch[1]: all common symbols, arrows, esc, ctrl, alt, forward delete, paste and enter gestures etc. etc.
[0] - https://jbak2.ucoz.net/ (yes it's horrible website, but it's free and easy to maintain for the handicapped developer)
[1] - https://github.com/Archargelod/jbak2Templates-Skins