My guess is that when Zelena took over OZ, Glinda tried to stop her and found out her magic wasn’t strong enough. Zelena then banished Glinda to FTL to prevent her from causing any issues in OZ perhaps after Dorothy tried to melt Zelena.
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Nothing is impossible on OUAT. Everything has a way of repeating itself. Keeper of the Keeper tread: https://oncepodcast.com/forums/topic/official-keeper-thread-3/#post-217924