Would removing syndication.php (therefore RSS functionality) cause any issues within forum?
Over the years I have seen multiple other forums stealing content via RSS. I want to remove that feature.
What is the best way to do it? Changing permissions to 0000 would help?
It doesn't look like it would cause problems. Disabling access to the file in .htaccess / nginx configuration would prevent issues during upgrades and File Verification.
Note that MyBB's default templates contain links to misc.php?action=syndication and syndication.php that should then be removed.
I've changed permissions to 0000, it seems that it stopped automated content theft. The last thread opened in my forum didn't appear there yet. Do you think that alone would solve the issue I am facing?
I have also changed footer and headerinclude templates, are there any other template I should be changing?
RSS is more easily machine-readable (and may be convenient for legitimate users), but ultimately the content will be available as HTML on normal forum pages (and archived in search engines, etc.). You may be able to identify and block some automated visits with manual IP filters or server configuration.
There are also references in forumdisplay ($rssdiscovery), and portal.
.... and headerinclude
<link rel="alternate" type="application/rss+xml" title="{$lang->latest_threads} (RSS 2.0)" href="{$mybb->settings['bburl']}/syndication.php" />
<link rel="alternate" type="application/atom+xml" title="{$lang->latest_threads} (Atom 1.0)" href="{$mybb->settings['bburl']}/syndication.php?type=atom1.0" />