I have a Wordpress site with Xenforo Forums on the same domain. The forums have over 200,000 threads and over 5,000,000 posts - they were once huge but have been dead for years. We've been debating to shut down the forums since they only account for 2% of traffic and are the most troublesome to host and maintain (98% traffic comes from the main site). On top of that, the forums were ported in 2017 from vBulletin to Xenforo, but it wasn't done properly and hundreds of thousands of 404 errors showed up in Google Search Console, new threads are barely getting indexed anymore etc.
Redirecting all forum URLs to a landing page on main site would fix hundreds of thousands of 404 errors in Google Search Console.
On the other hand, shutting down the forums would result in hundreds of thousands of new redirects and a lot of Google-indexed content disappearing. We'd redirect the /forums/ path to a new landing page on the main site to inform users of the shutdown (note: the forums are NOT hosted in a subdomain, they are on /forums/ path). I worry having this much content disappear would be a major red flag for Google and hurt our site rankings big time. So the worry is that it would mess up SEO beyond repair.
What are your insights on shutting down the forums from an SEO standpoint? Would it hurt the main site? And how would you go about doing it (e.g. 301 redirect to a landing page on main site)?