This is obviously just my opinion.

OS/AG/DP are clearly linked, we get to hear about the next series' region before the previous one ends and transitions between them are, let's say, smooth. Also, Ash actually progresses as a trainer and gets further developed as a character.

Things start to get weird with Black & White. Let's see: this is 5th gen, which some people like to see as an "americanized reimagining of gen 1" and Unova, the first region that's based on a place outside Japan. There are art style changes and for some reason, after over 600 episodes Ash becomes a newbie all over again and manages to look younger...

So, is BW a ("soft") reboot or what? Things get confusing as soon as characters from previous seasons start to make cameos since this should mean that BW is NOT a reboot by any means and continuity is assured, right? Well, I think not, they don't really make up for the fact that Ash's skill and personality gets reset to the point that previous seasons don't make a difference. They aren't enough to reject idea that BW is a reboot. But anyway, let's move on to the next series.

Ash gets better in XYZ and that's why some people claim it's a continuation of DP even though it couldn't be clearer that XYZ is a sequel to BW (just like AG to OS, for instance). If you consider BW to take place in a "different continuity", then XYZ does too.

Regarding Sun and Moon, just think of BW. It's the same stuff all over again: Ash looks younger, his skills as a trainer and personality get reset and there are changes to the art style. For me this is yet another different continuity, except this time he actually wins, kek.

Conclusion: there are three different continuities/timelines that may share characters and/or events. The first one consisting of OS, AG and DP, the second one being BW and XYZ and the third one SM. The "Ash running away from the camera" scenes at the end of both DP and XYZ (pic related) mark the end of a timeline.