Folks, before laying the badwrong on these people - remember that not everyone is good at framing a point or discussion well. This is not a "warning" sign. Yeah, it is nebulous, but then almost everyone is nebulous from time to time.
Such a statement is a cue for a conversation, not labelling.
I can only speak to my personal experience, but
my experience is that DMs/Storytellers/etc. who say they want to "ban powergaming" or something similarly nebulous and indicating a fear of powerful PCs or players who minmax, aren't typically interested in a constructive conversation, but rather a set of diktats directly from them (or worse, directly from some awful website or youtuber they follow), which are often extremely nebulous and inconsistent, and typically based in very serious misunderstandings/misapprehensions re: the rules of the relevant game system.
This is because they are
motivated by fear of the people they're playing with. It is difficult to have constructive conversations with people motivated by fear to start with. To have conversations when they're afraid of you? Not impossible but very hard. Particularly where the fear is
irrational bordering on paranoid, as often the case here.
This is why it actually is a warning sign/red flag and that's not just mean "labelling" or whatever.
Now, if a DM I knew well said something like "I'm concerned about too-powerful PCs" or something, that's the start of a conversation, so that can probably be steered to sanity and an understanding of the actual goal. But opening with "I want to ban powergaming" is a red flag, especially from a new DM.
It's a particularly red flag with 5E as 5E is
not a game where powergaming is a serious issue. Especially if you just ban multiclassing. As has been discussed many times, if the most powerful single-class PC is a 10/10, the weakest is probably a 7/10. Which is good work by WotC for sure. But some DMs live like it's 2003 and as if their players are all trying to create Pun-Pun. What they need is therapy and a nice cup of tea and maybe to be a player for a while, not to be crudely attempting to ban "powergaming".
What about a player who is good at minmaxing but consciously makes a poorly-optimized multiclass character?
I'll cross that bridge when I come to it, honestly. I doubt I'll ever come to that bridge.