Reasons to Restrict Multiclassing?

[2D8HP]

The DMG happily advises you to swap class features as an when you want.

Where is that at?
I saw Monsters as NPCs, and Villain Options with Death Domain and Oathbreaker Paladin...

I couldn't find it either with a quick skim of the DMG, but I did find:

"...As the Dungeon Master, You aren't limited by the rules in the Player's Handbook, the guidelines in this book, or the selection of monsters in the Monster Manual...

...Before you add a rule to your campaign, ask yourself two questions:

Will the rule improve the game?
Will my players like it?

If your confident that the smswer to both these questions is yes, then you have nothing to lose by giving it a try...

...No matter what a rules source, a rule serves you, not the other way around

Where have I seen something like that before?

Oh yes:

From Dungeons &Dragons vol. 3: The Underworld & Wilderness Adventures (1974)
page 36,

"AFTERWARD:
There are unquestionably areas which have been glossed over. While we deeply regret the necessity, space requires that we put in the essentials only, and the trimming will oftimes have to be added by the referee and his players. We have attempted to furnish an ample framework, and building should be both easy and fun. In this light, we urge you to refrain from writing for rule interpretations or the like unless you are absolutely at a loss, for everything herein is fantastic, and the best way is to decide how you would like it to be, and then make it just that way! On the other hand, we are not loath to answer your questions, but why have us do any more of your imagining for you? Write to us and tell about your additions, ideas, and what have you. We could always do with a bit of improvement in our refereeing."

Same game, same most important rule:
The DM makes the rules.