Question involving RAW. Basic question: does a multiclass druid know, and are they able to prepare and cast, any spell on the Druid spell list for which they have a spell slot? For instance, would a Wizard 19/Druid 1 be able to prepare and cast a 9th level druid spell?
It seems like one of the major limitations put on multiclass spellcasters in RAI is that their spells known list is determined by each class individually. So, for instance, as pointed out in the multiclass rules, a Ranger 4/Wizard 3 would only know second level Wizard spells (gained when leveling to Wizard 3).
However, druids seem to buck this limitation. It seems like any druid would know, be able to prepare, and be able to cast a spell of any level for which it has a spell slot, even via multiclassing. So, for instance, a 1 Druid/19 Wizard could prepare and cast a 9th-level Druid Spell.
Multiclassing rules, PHB pg. 164
"You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class."
Druid Rules, PHB pg. 66
"You prepare the list of druid spells that are available for you to cast, choosing from the druid spell list. When you do so, choose a number of spells equal to your Wisdom modifier + your druid level (minimum of one spell). The spells must be of a level for which you have spell slots.
So if I were a 1 Druid/19 Wizard, I would be able to prepare druid spells = 1+Wis modifier, and of any spell for which I have spell slots. As a 20th level multiclass caster, I would have the full slate of spell slots - so I could prepare 4 level 9 druid spells if I so chose.
Clarifying the decision spell selecting process for Druid. The rules for multiclass read that you determine spells you know and can prepare as if you were a single class character of that level. This refers to spells known (does not apply to druids) and can prepare (the number, 1+Wisdom modifier for the 1 druid/19 Wizard in question. Spell slots are explicitly generated by combining levels, and druids explicitly can prepare a spell of any level for which they have a spell slot.
Please point out the error in this interpretation of the RAW. I assume rule as intended would be "Spell slots you have available from your Druid class level".... but that's definitely not RAW.
This is tangentially addressed in this "Can Multiclass Wizard Cast any Spell", but only in the Wizard context (where there is more evidence for limited casting) and only by adding words to the rules not actually in the rules. So this answer does not appropriately address this question.