BA makes these trivial.To be honest disallowing multiclassing in 5E/1D&D does basically no damage to the game. There are very few genuine RP concepts which benefit from multiclassing, and 95% of multiclassing is either:
A) System experts exploiting synergies to attempt to make an OP character.
B) System noobs/ninnies picking "kewl" classes because they're allowed to and usually creating barely-playable junk characters full of anti-synergy.
Neither of those is a good things and that's the overwhelming majority of 5E multiclassing.