UIEnter executing in terminal

is UIEnter supposed to fire with neovim in xterm or gnome-terminal?
i put the following in $MYVIMRC : autocmd UIEnter * colo deus and colo deus is executed with neovim in the terminal

Yes. For TUI, v:event.chan is 0 on UIEnter, so you can do this:

autocmd UIEnter * if v:event.chan !=# 0 | colo deus | endif

I’ve opened a PR to clarify docs: https://github.com/neovim/neovim/pull/16804

what about GUIEnter ?

GUIEnter has been deprecated. It is never triggered.

why was it deprecated in favor of UIEnter ?

Oh, actually GUIEnter and GUILeave are not deprecated, as they never existed in Nvim. They only exist in Vim.