This is already the case.I'd turn feats and multiclassing off by default. Make them very distinctly an optional rule to the point of having them not available or restricted in AL play.
Rules for Adventurer's League are separate from the rules for D&D. It uses some optional rules and has its own house rules.
Feats and multiclassing are already presented as being optional and the majority of tables don't use them.
Keep in mind that AL probably represents 1% or less of the 5e player base.