Pollution would be any undesirable side effect and there are plenty to be imagined in these scenarios.
Your comment strikes me a bit like a chemist denying environmental pollution as it's all "just chemicals."
Chemicals admit of a form of pollution when they form themselves in ways we don't like. Spacetime doesn't have that sort of memory, excepting exceedingly weak gravity waves, for which it is basically impossible for those to ever be your biggest problem (if not simply impossible for those to really be a problem). There's no spacetime equivalent to dumping a nasty chemical somewhere, and a hundred years later that chemical is still there hurting people. Spacetime doesn't work like that.
You're basically anthropomorphizing, thinking that there just has to be a way for humans to screw this resource up because humans are just that icky and gross, because cynicism and fashionable misanthropy. There isn't. Not in the real math. There's nothing humans can do in the real math that we know that the universe hasn't already done a couple dozen orders of magnitude larger than we could ever dream of. If space was going to "rip" or something, it would have already. It doesn't. In fact, in the real Einstein field equations, space very, very, very aggressively doesn't rip or suffer any other such problems. Any attempt to introduce such defects results in those defects immediately (and often quite energetically) undoing themselves. That's why when you read about real wormholes, for instance, the story always ends with the wormhole collapsing and exploding before anything, even light, could possibly traverse it.
Real math may change, in which case this answer may change. However, I expect that by the correspondence principle, even the ultimate unified theory of everything is unlikely to have an outcome where suddenly all this stuff is possible. Whatever the ToE is, it's still going to reduce to General Relativity in the large scale.
Maybe such technology involves the creation of unnatural structures, something like stable wormholes on human scales. If those structures outlived their creators, areas of space containing them could be considered polluted, or at least dangerous to later civilizations.
Douglas Adams did some thinking on this for Hitchhicker's, that time travel caused cultures to stagnate and eventually degrade for lack of motivation.
death knell
I know you probably know this, and I don't mean any disrespect. I'm writing this for other people who don't speak English natively (I'm also not saying you're not a native speaker, just that I'm not). I don't want anyone to go away with the wrong idiom.
I said nothing of the sort. You are, perhaps, confusing me with another poster.
"There's no spacetime equivalent"
Nonsense. The obvious example is emitted radiation, though any type of energy, matter or field byproduct might fit the bill.
The idea of "polluting" spacetime makes no more sense than humans "polluting" the natural organization of air molecules by displacing them by existing, or polluting the natural distribution of entropy by making decisions.
It's just bringing politics into a place where it doesn't belong.
"the presence in or introduction into the environment of a substance or thing that has harmful or poisonous effects."
I have no idea why you have this idea that polluting is some politicized term.
Your argument also doesn't hold considering that in the case we're talking about right now, the vast majority of spacetime is not inhabited nor is it regulated by anybody. Yet we're still able to talk about pollution because pollution in no way is tied towards a politicization of the conversation.
One could even go so far as to say you've polluted the conversation by including this "but you're making it political" distraction.
It's true that it isn't a problem -- until it is.
We are able talk about harm being done to insect populations or coastlines or any other inanimate objects. Again, humans, and their politics, aren't necessary.
I don't find my quoting of the definition as being petulant. I was wanting to frame the conversation around a shared definition of the word rather than asserting requirements.