Lemmy’s license is AGPL, so you would need to at least publish changes to Lemmy itself 😉

(I don’t know if e.g. the code for the algorithm is separate, in order to have a closed source algorithm with an open source Lemmy fork)

There is also zellij, which can do the same but also has modern functionality specific for development workspaces!

(Although screen or tmux will still probably be more widely available on remote machines etc)