There's this thing called the internet archive a.k.a. wayback machine that someone has to trigger each time they want a web page to be saved
Here's the page to trigger a save
If you select save outlinks it will also save pages at the end of *1* link hop away. Since in a long thread these forums provides links to the first page "<<", last page ">>", 5 pages to the left and 5 pages to the right (the numbers you can click on to take you to another page) for maximum efficiency start saving at page 6, then increment by 11 pages (6,17, 28, 39, 50, 61, ...) if your numbering is different, I'm describing the desktop view which ironically I only get on my smartphone when I select desktop view in the browser... but the info is there even if not shown by the cascading style sheets your browser is using.
Each page of a forum also provides links to the first few/last pages of all threads on that page so in hindsight if some one had been triggering a save on the first page of each forum/sub forum once a night we'd have most of everything already... but it also provides a way to back up short threads without visiting them.
There's a lot of us, a lot of threads, and enough time left that we can back up most of these forums before they go away, if we get organized.