I made a really scientifically accurate study and asked opinion of my few active users and of my friends who aren’t on my forum. Any of them are aren’t IT/web-professionals, but ordinary end users.
Any of them didn’t even understand point of such icon and they had hard time to understand whole question. I suppose it is because such system isn’t in use anywhere that is familiar for them, aka. social media. If you are a member or otherwise allowed you can see content, if you aren’t… well, you can’t see. Like everywhere.
So, the issue of padlock-what-ever icons must rise from admins’ need to advertize. Then there is need to show some indicator (like paywalls should do when sharing links) only when such closed or limited topics or categories are visible on heading level. But that is the only reason and then what ever mark works.
This isn’t a general UX topic anymore, if even has been. This is topic of how closed and paywalled forums should warn theirs random users not to make unnecessary clicks. Another edge case.