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I think at this point The Pokemon Company will treat it on a case-by-case basis so there will be games Mega Evolution will be seen and games there will be nothing of the sort.
A meta-reason for the existence of Mega Evolution was to give older Pokemon a new life on the competitive scene in Gens VI and VII, but seeing how it ended up being so gamebreaking was why it was omitted from Gen VIII.
I don't think Mega Evolution was quite so gamebreaking in Pokemon Super Mystery Dungeon which is why it returns in the Rescue Team DX remake. It's to give some lore to why Mega Evolution exists in PSMD by making it appear in Rescue Team DX.