The best practices require a 404 error page in this case. You could create a friendly custom 404 page to avoid losing visitors. I understand you don't want to lose any incoming links, but pointing dead links to your home page isn't what redirects are for.

hi i don't mind losing visitors. The only question is about seo and links.
thank you

the problem is that i generate lots of 404 errors, and all links pointing to the forum threads, will be lost.

should i 301 all /forum/.... threads to the www.domain.com homepage?

No. No, a thousand times no. Happily, search engines and humans agree on this: redirecting to the home page benefits nobody.

If you are intentionally removing content, that warrants a 410. Make a nice custom 410 page, which will be mainly for the benefit of humans.

If links point to specific content in the forums, and that specific content is now gone, there's really not much use trying to hang on to the links. It's not a case of moving content from one URL to another.

hi i don't mind losing visitors. The only question is about seo and links.

when your visitors follow those links and get redirected to your home page, most will likely bounce and thus they are essentially lost.

if your seo question relates to google:
when googlebot requests all these /forum/ links and they are all redirected to the home page, the "soft 404s" [webmasters.googleblog.com] will eventually be recognized as such.

the "soft 404s" will eventually be recognized as such

As soon as Google meets a lot of redirects--regardless of circumstances--it will programmatically start requesting garbage URLs in the form
/ugfjgdfcvkjfbmhjnkbjdfkjg.html (I-forget-how-many random letters, always at the root)
If these, too, meet a 301 to the home page, you are definitely in "soft 404" territory. I suspect it is not quite as catastrophic if only selected requests get redirected to the home page--but again, for the sake of your humans, why would you want to?

First: thanks everyone for your replies

the goal is to completely wipeout the forum and leave only the www section. (not www.domain.com/forum/...)
The main problem is that forum contributes with thousands of backlinks to the entire domain and i won't lose them of course.
I did not understand yet what exactly i should do in order to delete /forum/ and keep backlinks.
For example: a link pointing to www.domain.com/forum/thread1 should be mantained but i don't know how to transfer its seo juice from it to the main domain, that's why i thought about 301 all of them.
I assume If i simply remove them or 410 them, etc.. they will be lost.

anyone?

if your inbound links to /forum/ have any value currently it is because they are linking to relevant content.
when you redirect all those thousands of links to the home page, they lose that value unless the home page is still relevant to each link, which is extremely unlikely.

your choice is to keep the relevant content, create new relevant content and 301 redirect the old urls to the equivalent new urls, or respond appropriately to the request by sending a 404/410 status code.

i would suggest in any case you should regularly monitor your properties on google search console so you are aware of any issues or trends as they occur.

the problem is there are tens of thousands of threads and it is impossible to work on them unless wasting months of work
anyway thanks for your help

How long would it take to replace those thousands of inbound links?

they are about 300 inbound links (from 300 different domains...) and i would like to keep them

i don't know how long it would take and i don't know how and what to tell the forum platform (mybb) in order to keep the flow up to the main domain.
that is the main problem.

Let's put it this way: If you remove content, you cannot expect to hold on to benefits that were the direct result of that content.

ok thanks