Posted August 22, 2023
The "known spells" are indeed decided by class level, for Bard, Cleric, Druid and Sorcerer. As are the Wizard in regards to what spells they learn at levelup, too. But Wizard also can learn from scrolls, and by the rules of the wizard spellbook, they can learn up to their maximum spell level spells. Which is Larian Studios interpretation.
Or one could say no the spellbook rule only refers to spell levels the wizard has access to by virtue of their wizard level, which is your interpretation. It clearly doesnt say so in the section about the wizard spellbook though and the formulation in the other section frankly isnt clear enough.
Its the same situation as with Cantrips.
Firstly, we can't know whether Larian interpreted the PHB a particular way or not, unless you happen to work there or have asked them, regardless of the implementation, as they have a number of other, overt homebrew rules in BG3, such as removing the multiclass ability score requirements. Although it would be a great question to ask them in an AMA. Having lived and breathed the PHB for the last 6 years, they must have a solid opinion.
Secondly, the multiclass rules rather painstakingly specify 'know and can prepare' rather than just 'know'. The Wizard class description at https://5thsrd.org/character/classes/wizard/ has a specific section entitled 'Preparing and Casting Spells' stating
"The Wizard table shows how many spell slots you have to cast your spells of 1st level and higher.
...
You prepare the list of wizard spells that are available for you to cast."
That table, as expected, only allows a Wizard 1 to prepare and cast 1st level spells. Therefore, in line with the multiclass rule of 'You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class', a Wizard 1 cannot prepare higher level spells. It's irrelevant that there's another multiclass rule granting higher level spell slots because for the purpose of preparing spells, the rule explicitly states you have to consider the Wizard 1 individually. Individually, the Wizard class would not have higher level spell slots.
Ultimately, though, in a way for me the biggest argument is the thematic one. It just doesn't make sense for a novice Wizard 1 to be able to cast higher level spells.
Lastly, it turns out Jeremy Crawford has actually covered this topic explicitly: https://twitter.com/JeremyECrawford/status/941083631388213248
Post edited August 22, 2023 by twistedpony