There is no :shell command in neovim.
:terminal opens a shell without previous output, while :shell contains all the output of the parent shell and all previous shells. Is there any way to emulate :shell behavior with :term?
As @statox mentioned, you can use CTRL-z. If you want to spawn a new shell from vim, as :shell does, you can do: :!sh, :!bash, :!zsh (pick your favourite), which has pretty much the same effect as :shell.