It’s giving me the recently boosted characters cannot access legacy dungeons and raids. 
This isn’t legacy if you can do it on M+ :-/
It is considered legacy if it is not a dungeon from the current expansion. There is a rotation of legacy dungeons that can be done on M+ but they are still legacy.
No matter if indeed it is a legacy content, if it is featuring in the current patch, with new mechanics and so, dev team should have foreseen that happening.
I had all my gear ready for the new toon with some BoE and legendaries and cant join most of the M+ content because of this bug. Even if it is for a short period of time (not sure about that one) it dosent make any sense.
Im adding this with the new toon for no particular reason, but I have a mythic keystone here for Iron Docks and cant join my own stone. It is indeed obviously a bug 