This being said, it should be clear that a wizard 1/cleric 19 having access to all cleric and wizard spells can't be the intention.
Well fortunately it doesn't work that way.
1 Cleric / 11 Sorc can only cast level 1 cleric spells.
The game is combining total spell levels for arcane or divine levels, in fact NWN1&2 already did this - 'Your arcane / divine spellcasting raises every level / every other level / every level beyond first'.
Seems to be working as intended. If you think it isn't send a support email.
I was talking about how the 5E P&P rules work vs BG 3 rules work. For PnP it is correct that for spell slots you add up total caster classes - so a Wizard 10/Cleric 10 does have level 9 spell slots. He just can't prepare spells of that level, so they can only be used to upcast lower level. In BG 3 on the other hand, he can also learn and prepare them, which is an issue with the game.
I know how 3.5 worked.
Didn't realize thats what you meant, I've seen more posts here explaining this that spell slots in pnp 5e are based off total caster levels and not total class levels, but OP seems to not care about that, plus as I edited into my last comment, homebrewing of rules exists even if people are adamant that something is not an official ruleset feature.
In fact I've not yet seen anyone else so fiercely defend pnp rules to the point of 'everything and everyone else is 100% wrong, I am the only one that's right'.
If OP makes the claim calling something broken and against pnp rules, they also invite discussion to explain that actually no it isnt.
The DM can do whatever the DM wants, they can make up rules as they see fit, e.g. if they don't like players always using the same spells again and again, they can ban fireball. They can give wizard players a specific list of spells that they are limited to using., i.e. this guy's videos
