But the DM does also have control over mechanics because of how it effects the game. And this is both for in-world reasons and for non in-world, play reasons.
Yep. But if a DM disallows any class for whatever reason, then that class is equally unavailable for a MC PC.
Therefore ANY abilities possessed by a MC PC are already allowed by that DM in that world.
Maybe a DM says "The gods of fickle whom they grant power - you can't MC warlock with either paladin or cleric in this world". Even though warlock, clerics and paladins all exist, there can be in-world reasons that disallow the multiclass.
For a particular MC combination, perhaps. But no god is going to take his favour away on the grounds that you just learned to fight better, so the idea that a god would abandon you for taking a fighter level is absurd.
Remember that the thing that I'm railing against is a blanket ban on MCing without any justifiable reason for a blanket ban, either in game mechanics terms or in 'vision of the world' terms.
Let's go for a non in-game reason. Say a DM wants to include a number of UA classes/subclasses (or DM's Guild, or whatever) but doesn't feel that they are as balanced vs. cherry-picking as the great job done on the PHB classes. (Mearls has even said that multiclass balancing comes at a later point then the first UA draft.) Saying "once you take a UA class you need to go to at least 3rd before you can take levels in anything else". Right there you have a DM doing due diligence (justified or not) for allowing playtest/3rd party products in to allow the players more choice.
It is wise to be cautious of UA stuff, and a DM can be honest and say that he wants to test UA classes but doesn't want the test polluted/exploited by ill-thought-out new interactions. I've no problem with that.
But a blanket ban on MCing with PHB material simply doesn't have the same excuse.
There are literally decades of D&D play in earlier editions that say that armor messes with magical physics and arcane casters can't wear it. Blanket prohibition. And even when it start being allowed, there was a failure chance that you'd just lose the spell. This was as real in-game as "metal is rare and valuable" is in Dark Sun.
Absolutely! Introduce the mechanic: spell failure chance/cannot cast spells in metal armour, go for it! But don't pretend that this mechanic prevents MCing! It might influence the choices, but not make MCing impossible. I'll just have a level of fighter but be Dex-based and use
mage armour.
We also had clerics unable to use any weapons except bludgeoning ones regardless of who the worshiped, and we STILL have Druids who can't/won't wear metal armor.
Again, go for it! But don't pretend that this has
anything to do with MCing; these weapon/armour restrictions apply to single class PCs too.
There's plenty of reasons why a DM customizes their own game for the setting, campaign, and mechanical feel they want. Including at the mechanical level that's not observable in-game. There should still be player buy-in - DM & players all come to have fun. But having the rules support the setting and theme for a particular game makes sense to me.
But the MC rules do not impact the list of allowed classes or allowed class abilities at all!
If someone said that it was
compulsory for every single PC (above 1st lvl) to be multiclass, I'd be berating them too! It would make just as little sense as making single class PCs compulsory (which is the effect of disallowing MC). It wouldn't make sense in 'world vision' or 'allowed abilities'.