And that is just another factor for the "Spellcaster supremacy" claim.
It doesn't matter if I'm on the agreeing side, you still need to be specific.
So I was quoted Mizzium Apparatus's rule text, take it with a grain of salt since this is second hand info but.
Innovation is a dangerous pursuit, at least the way the mages of the Izzet League engage in it. As protection against the risk of an experiment going awry, they have developed a device to help channel and control their magic. This apparatus is a collection of leather straps, flexible tubing, glass cylinders, and plates, bracers, and fittings made from a magic-infused metal alloy called mizzium, all assembled into a harness. The item weighs 8 pounds. While you are wearing the mizzium apparatus, you can use it as an arcane focus. In addition, you can attempt to cast a spell that you do not know or have prepared. The spell you choose must be on your class’s spell list and of a level for which you have a spell slot, and you must provide the spell’s components. You expend a spell slot to cast the spell as normal, but before resolving it you must make an Intelligence (Arcana) check. The DC is 10 + twice the level of the spell slot you expend to cast the spell. On a successful check, you cast the spell as normal, using your spell save DC and spellcasting ability modifier. On a failed check, you cast a different spell from the one you intended. Randomly determine the spell you cast by rolling on the table for the level of the spell slot you expended. If the slot is 6th level or higher, roll on the table for 5th-level spells. If you try to cast a cantrip you don’t know, the DC for the Intelligence (Arcana) check is 10, and on a failed check, there is no effect.
Wondrous Item, uncommon (requires attunement by a sorcerer, warlock, or wizard)
Your capacity for spellcasting depends partly on your combined levels in all your spellcasting classes and partly on your individual levels in those classes. Once you have the Spellcasting feature from more than one class, use the rules below. If you multiclass but have the Spellcasting feature from only one class, you follow the rules as described in that class.
Spells Known and Prepared. You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. If you are a ranger 4/wizard 3, for example, you know three 1st-level ranger spells based on your levels in the ranger class. As 3rd-level wizard, you know three wizard cantrips, and your spellbook contains ten wizard spells, two of which (the two you gained when you reached 3rd level as a wizard) can be 2nd-level spells. If your Intelligence is 16, you can prepare six wizard spells from your spellbook.
Each spell you know and prepare is associated with one of your classes, and you use the spellcasting ability of that class when you cast the spell. Similarly, a spellcasting focus, such as a holy symbol, can be used only for the spells from the class associated with that focus.
Spell Slots. You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, half your levels (rounded down) in the paladin and ranger classes, and a third of your fighter or rogue levels (rounded down) if you have the Eldritch Knight or the Arcane Trickster feature. Use this total to determine your spell slots by consulting the Multiclass Spellcaster table.
If you have more than one spellcasting class, this table might give you spell slots o f a level that is higher than the spells you know or can prepare. You can use those slots, but only to cast your lower-level spells. If a lower level spell that you cast, like burning hands, has an enhanced effect when cast using a higher-level slot, you can use the enhanced effect, even though you don’t have any spells of that higher level.
For example, if you are the aforementioned ranger 4/ wizard 3, you count as a 5th-level character when determining your spell slots: you have four 1st-level slots, three 2nd-level slots, and two 3rd-level slots. However, you don’t know any 3rd-level spells, nor do you know any 2nd-level ranger spells. You can use the spell slots of those levels to cast the spells you do know—and potentially enhance their effects.
A spell scroll bears the words of a single spell, written - a mystical cipher. If the spell is on your class's spell list you can use an action to read the scroll and cast its spell without having to provide any of the spell's component. Otherwise, the scroll is unintelligible.
If the spell is on your class's spell list but of a higher level than you can normally cast, you must make an ability check using your spellcasting ability to determined whether you cast it successfully. The DC equals 10 + the spell's level. On a failed check, the spell disappears from the scroll with no other effect.
Once the spell is cast, the words on the scroll fade, and the scroll itself crumbles to dust.
The level of the spell on the scroll determines the spell's saving throw DC and attack bonus, as well as the scroll's rarity, as shown in the Spell Scroll table.
So a Wizard 19/ Cleric 1 as the Spell Slots of a 20th level spellcaster. They can become attuned to Mizzium Apparatus and their Spell List technically includes both Wish and Mass Heal for example. However they do not know any 9th level Cleric Spells and cannot prepare them which is the only limitation provided by the PHB. So it comes down to how you think "your class’s spell list" should be interpreted (and scrolls favor one of these). On the one hand it just makes Wizards massively versatile. And on the other, it does the same thing but better.
Edit - It was brought up so here is something else to think about. If a Divine Soul Sorcerer chooses Bless it becomes a Sorcerer Spell to them and it's part of their Class List
. If at the next level they choose to replace bless with lesser restoration then lesser restoration becomes a sorcerer spell to you. The text does not remove Bless's state of being a Sorcerer Spell (and Crawford even uses past tense), only that you no longer know it. It's possible for a Divine Soul Sorcerer to cherry pick multiple Cleric Spells to construct a very large Spell List of unknown and Unprepared Spells.