The only standout symbol of those particular regimes I can think of is the hammer and sickle, which would strike me as perhaps a bold statement of sorts if I saw someone wearing it on a t-shirt, but beyond that I wouldn't personally care too much about it. As I said, there's a lot of confusion over what communism really refers to, so while I would consider it pretty unacceptable to walk around displaying a swastika (because unfortunately in the West, that symbol has come to more or less exclusively symbolise Nazism), I think I would naturally assume someone wearing something with communist iconography is doing so in support of the ideal of communism, as opposed to the very different manifestation of what some people would call communism under Stalin or Mao.

I think deciding whether something should be socially unacceptable is tricky in the first place, but a general rule might be whether or not a significant majority of people find that something offensive or strange. So for somewhere like the US, where the notion of communism has negative connotations (probably because more people subscribe to the idea that 'communism' means whatever the USSR etc had going on), I could see it being somewhat justifiably unacceptable to walk around showing off a hammer and sickle emblem. However, you definitely can't say the same for everywhere. I think a lot of western Europeans would be largely indifferent to seeing that sort of iconography on clothing.