General rule on these, content the item comes from, you need to be at least high enough level to get started in that content.
Vanilla raid stuff works on level 1s as they are in the range for Vanilla content, 1-60.
Burning Crusade appearances then unlock at 60 and so on.
It’s not exact as there are a few things that don’t follow that, but it’s a good place to start understanding some of the level limits.
As for the suggestions about the level limits themselves, that’s certainly something the WoW Team will need to look at going forward for Shadowlands with the change in the levels that’s coming.
Be interesting to see how that all works out 