This is a complicated situation, because I asked Shabook what to do back in May 2018, and he just said it's not top priority and is yet to give any updates.
There are two ways of doing this:
1) We have the 2018 page with columns/sections for different timelines (Agents of S.H.I.E.L.D.) and different versions of timelines (Runaways). Then for the post-2018 continuations of those timelines and versions of those timelines, they get their own pages: 2021 (Gert Yorkes Deceased Timeline Version), 2028 (Gert Yorkes Deceased Timeline Version), and 2091 (Lighthouse Universe/Timeline). Or just a Post-2018 (Gert Yorkes Deceased Timeline Version) page and a Post-2018 (Lighthouse Universe/Timeline) page.
This would mean that the pages can have the header description like the current "This is a timeline of events that occurred in an alternate 2021, before Gert Yorkes' life was saved during the final battle against Morgan le Fay, preventing this future from coming to pass." The pages are purely pages FOR those timelines and timeline versions.
In this case, the 2018 page would, in the "After" section, have to link to 2019 and the other two pages.
Alternatively:
2) We have a 2018 page, followed by 2019, followed by 2021, followed by 2028, followed by 2091. And on these pages, we have sections similar to 2018 (just not needing columns because there's nothing to put in the other column), saying it's part of a different timeline version or timeline. So the 2021 page would just say what a normal page would say, "This is a timeline of events that occurred during 2021." We could end up with other 2021 events at some point, with flashbacks to the Snap period or something, and you have them share a page, just with sectioned off non-main-timeline stuff like 2018.
I prefer Option 1, I think. The reason we did the column thing on the 2018 page is because it's important to show the shared timeline events up to the split point, and then good to see the side-by-side, and not worth it for its own page. If you had a separate 2018 page for the Lighthouse Timeline, it would be weird because either you have to make sure everything up to May 31st is identically copied-and-pasted to that page and also identically edited whenever anything January 1st-May 31st is edited from there on, but then also it would still be weird because it's not just identical events but it's literally THE same events in the same universe, with the universe only splitting on May 31st, or the page just begins halfway through May 31st which would also be weird.
But as far as I'm concerned, after those initial split points, they're separate enough, they've spun off. It would be weird to have completely separate events, say, decades later, side-by-side.
That said, there's also something to be said for Option 1 for Agents of S.H.I.E.L.D. because it's a different universe, but Option 2 for Runaways because it's just different versions of the same universe/timeline... I don't know. I'll ask Shabook or Nerdtastic about it again some time soon.
I think for now, since the 2021 and 2028 pages are solely devoted to Runaways events at the moment, it's OK to just consider them as pages for different timeline versions like Option 1, and indeed link 2018 to 2021 with "(alternate timeline)" or something and 2019 to 2023, and not just 2019 following into 2021 following into 2023.