With the upgraded ALL Accor+ Explorer programme, the Stay Plus FREE Night benefit has seen some improvements, so it's no longer subject to or limited by allocation.
Just a quick note, though: the new Stay Plus FREE Night benefit does require a minimum stay of two nights, with at least one of those being a paid night. And only one Stay Plus Night can be used per stay, a stay being consecutive nights in a hotel
Please note: Blackout dates are applicable and can be found here: Accor Blackout dates