Hi everyone. I just wanted to share this in case others in my situation can benefit somehow from this. I have a large forum with many millions of posts. Since the last 2 months or so the forum was very slowly losing organic traffic. I made some changes about 10 days ago and for the last week might traffic is up 15% or so and still going up every day. I can't be sure this is permanent, but in any case methods here are just good housekeeping and probably won't hurt your forum/website anyway.
I had a large off topic section. I trimmed it big time. All pages had noindex in the off topic. But anyway I kept the last 2 years of threads and the rest I removed from the off topic.
I created safe outbound domain list then looped through every single post with external links and removed any link not flagged as safe.
For internal links I made sure the broken links are fixed, the redirect are fixed and are pointing to the canonical form of the url.
Before all these I also switched the forum to https urls.