I think a better solution than to "go offline" is to just turn the whole site into static content and leave it online forever.
I have been running a music discovery website for a similar timeframe as Watmm now:
And for about 10 years, it also had a forum:
https://www.gnoosic.com/discussion/
It was just too much work to keep the forum running. But I can't imagine removing it from the internet. I know that some people are still happy to find their old posts every now and then and read what they wrote back then.