Magic: The Gathering lore in D&D questions
If someone asks about D&D lore with the dungeons-and-dragons tag, and someone answers with Magic: The Gathering lore that accurately answers the question, more power to them, I think. I think if it were me asking the question, there is a good chance that such an answer wouldn’t be the one I accept—but I probably would upvote. Obviously, it would be a better answer still if they connected the Magic lore to other D&D lore, or referenced D&D Magic products, but I don’t think either is absolutely necessary.
Questions about Magic: The Gathering lore
Note that we do not have any questions of this sort yet, and we should do nothing until we do. Still, some thoughts on how to handle it if such questions get asked.
If someone wants to ask about Magic: The Gathering lore, they should use an appropriate tag—probably magic-the-gathering to emphasize we’re talking about it within the context of D&D. (Magic does refer to its setting as “the multiverse,” but that name is far too generic to use for this purpose.) Such questions could probably also be asked at Board & Card Games, but it’s OK for a question to be on-topic at two different Stack Exchange sites—being on-topic there doesn’t automatically make it off-topic here. Of course, questions about the actual cards and rules of Magic: The Gathering are and always will be off-topic here and should be migrated to Board & Card Games.
I don’t think it would be necessary to use dungeons-and-dragonsmagic-the-gathering for questions about Magic lore; that combination should probably be reserved for questions dealing with how Magic’s planes are embedded in D&D’s multiverse, campaigns traveling between Magic planes and traditional D&D planes, and so on. On the other hand, we do tend to use dungeons-and-dragonseberron et al. for other D&D settings, so maybe we should.
Questions about individual Magic: The Gathering planes
The problem with dungeons-and-dragonsmagic-the-gathering then becomes, if someone wants to ask about the lore of any particular Magic plane, like Ravnica or Zendikar, that would imply they should use dungeons-and-dragonsmagic-the-gatheringravnica which is super problematic when there are only 5 tags allowed on a question. Tags should not be “nested” like that. I would be inclined to say they really ought to just use ravnica for that—but again, that’s inconsistent with how eberron and the like are used.
Honestly, it’s beyond the scope of this discussion, but I’d actually push for eliminating “redundant” uses of dungeons-and-dragons with D&D setting tags. Probably deserves its own question.
Until then, though, since we have no magic-the-gathering and no real need for it, using dungeons-and-dragonsravnica is fine.