Or was it an alternate timeline? I hate these things. Thanks.

I think It happened on 616.. then Thor erased(or whatever he did) to correct it. making the reigning an alternate universe.

I see it the same way that the Days of Future Past X-men future is looked upon. It is a future that could have happened but was averted but whether that was an actual 616 timeline seems unclear. Things are never clear cut where timeline jumping and changing the future are involved. Even alt timelines still happen. It could be that the Reigning was a 616 timeline and once it got averted in Marvel's standard continuity that became a new timeline.

It did then thor went back in time and changed the past so it did not

It did then it didn't. 616 Odin saw it, 616 Thor lived it and then Thor / Designate retconned it. Thor is the only one that knows about it in 616, but who knows if he still remembers that life post Ragnarok / the knowledge of all his other lives.

Pretty much this, though any "future" storyline should probably be viewed as an alternate timeline as any change in events preceding it throw all subsequent events into doubt.

It is 616 canon but since it was erased usually feats from it are not respected. Thor having lost the ability to wield his hammer, he single handedly (literally) kills hulk, thing, and just about every hero on the planet. While mind you being weak enough that Wolverine was able to cut off his arm. So you can see how consistency wise feats from the reigning are generally not respected.

But since it technically did happen and Thor just reset time it is still 616 canon, if the alternate universe persisted than it would be a different story, but as far as I know there is no other designation for the reigning timeline, which makes it 616 canon.

It happened, then Thor went back in time and basically made it so that it didn't happen. Its like erasing it from history and starting over

Interesting way of analyzing it, and if it hasn't been given a separate designation guess it still would be considered 616, or it's simply been overlooked up to this point.