The 2024 rules on learning new spells:
Whenever you gain a Wizard level after 1, add two Wizard spells of your choice to your spellbook. Each of these spells must be of a level for which you have spell slots, as shown on the Wizard Features table.
Q1: Does Multiclassing
rewrite this to remove "as shown on the Wizard Features table[/b] and replace it with
as calculated by the multiclassing rules? Ie, is the "as shown" a clarification, or a restriction?
You prepare the list of level 1+ spells that are available for you to cast with this feature. To do so, choose four spells from your spellbook. The chosen spells must be of a level for which you have spell slots.
Q2: This doesn't mention "as shown on the Wizard Features table".
Ignoring multiclassing, suppose there was a feat or race that gave you a 3rd level spell slot at level 1. A single-classed wizard would be able to prepare 3rd level spells that are in their spellbook; they wouldn't be able to scribe such spells when they gained levels, however.
Copying a Spell into the Book.When you find a level 1+ Wizard spell, you can copy it into your spellbook if it’s of a level you can prepare and if you have time to copy it.
If you had a 3rd level slot as a single-classed wizard, you could copy a 3rd level spell into your book (if you found it), then prepare it.
....
So, then the question is, when you prepare spell as a wizard, do you ignore the fact you have higher level spell slots from multiclassing and pretend your only spell slots are from being a wizard, and from that (fictious state) determine your max spell slot?
One could argue they should say "you can prepare spells up to 1/2 of your wizard level, rounded up" instead of mentioning spell slots; that would solve this problem. They don't, however.
This wording is similar to Sorcerer in 2024:
The number of spells on your list increases as you gain Sorcerer levels, as shown in the Prepared Spells column of the Sorcerer Features table. Whenever that number increases, choose additional Sorcerer spells until the number of spells on your list matches the number in the Sorcerer Features table. The chosen spells must be of a level for which you have spell slots.
And for changing:
Changing Your Prepared Spells. Whenever you gain a Sorcerer level, you can replace one spell on your list with another Sorcerer spell for which you have spell slots.
Which also relies on "have spell slots".
"Up to half your level, round up, and no more than 9th" would solve this, or an entry in the table.
Only the wizard mentions "look at the wizard table" for the source of slots.
...
Ok, balance impact:
A Paladin 2/Sorcerer 10 can prepare 5th level Paladin spells, sooner than a pure Paladin can. I don't really like that.
A sorcerer 10/wizard 10 has access to 9th level spells for both sides.
Are the higher level class features of "pure casters" worth not multiclassing under these rules? Admittedly, multiclassing pure spellcasters was pretty dumb in 5e due to the loss of high level spell options.
But, access to 9th level bard spells by splashing bard 1 also seems sus.