It's theoretically possible to be literally true, but I'm not aware of anybody putting forward any testable hypotheses related to the notion. Until that happens it's philosophy and or some form of pseudoscience.

That being said, just because it's not science at this time doesn't mean that it can't be a useful model for dealing with the lack of coherency in the world we're living in. Pay close enough attention and you'll start to notice that things don't maintain a static history the way that you would expect. New information comes out that causes reinterpretation of past events all the time.

About alternate universes, realities or time lines. Considering they technically don't exist in ours wouldn't it be impossible to prove that they even exist?

Most likely, but that would really depend upon the specifics. I wouldn't count on it ever being settled conclusively. At this stage I wouldn't expect it to be testable any time soon.

There's also the issue of identifying whether it's the universe itself that's inconsistent, our flawed observation or just a complicated enough set of rules to make it worthwhile.

That being said, it isn't necessarily the case that we can't test for it, it's just that nobody has figured out how. Multiple time lines and realities would likely leave at least some traces on what we know. And there's an awful lot of weird stuff that happens.