Multiclassing is in theory an optional rule, but I have seen it at 100% of games that I played.
So, in my experience it's a core rule. Same as feats.
Problem with multiclassing is that classes need to be defined early on.
That is have lot's of features at levels 1-3.
early on, that is not a problem, most of the time 6th level single class is better than 3/3 split. but 5/1 could be "cheesed" into some OP combo.
at 10th level 5/5 split will mostly be horrible compared to 10th level character, but 8/2 or 9/1 or 7/3 could again be a problem.
also, to me, even split of two classes would be perfect representation for multiclassing.
And only two classes.
within one level of eachother.
But, how to deal with the power problem of missing higher level features?
solution could come from 3.5e multiclass feats from Complete adventurer/scoundrel.
I.E.
feat: raging shifter:
requires 3 levels in barbarian and druid.
+1 ASI,
Your barbarian and druid levels stack for calculating rage damage and number of usages.
Your barbarian and druid levels stack for calculating wild shape CR and types used.
You can enter rage and wild shape as a same Bonus action if you are moon druid.
some class feature might be too strong so classes could only add half(round up) of their level to other class faetures.
I.E.
Feat: martial stalker:
requires 3 levels in fighter and rogue.
+1 ASI,
Half your fighter levels(round up) add to your sneak attack progression.
half your rogue levels(round up) add to your fighters Extra attack progression.
feat: mystic theurge:
+1 ASI
requires 3 levels in two full casting classes:
Half levels in one class adds to levels in other class for availability of spell levels.
Spells prepared and known stay the same.
that is 6th level wizard/6th level cleric with this feat would both have available 5th level spells as both classes would count as 9th level(6+3) for access to spells.
both classes could still prepare only 6 spells plus their respective casting modifier.
Spell slots would be of a 12th level caster per normal multiclass rules.