I think Electrum in some previous release, do not make opt-in RBF to be default. Later, opt-in RBF has been by  default on the newer update releases since many months ago. On desktop electrum, you can still be able to uncheck opt-in RBF on preference as hosseinimr93 posted, also while on mobile Electrum, no more on setting, but you can check a transaction not to support opt-in RBF (anytime you are making a transaction). Not checked by default.

The only reason I think Electrum developers can remove opt-in RBF is only when almost all nodes support full RBF. Maybe this would be possible if full RBF is set to default on Bitcoin Core which may occur next year as o_e_l_e_o predicted.