I have posted numerous homebrew features on Role-playing Games Stack Exchange, and I have used advice gotten from that site to balance most of them. If, one day, I publish the subclass, would I be required to give any or all of the following credits?
To the RPG.SE website as a whole
To the individual users that gave feedback
Just to the users whose feedback I used
Or would I not be required to give any credit?
First off, you own everything that you originally created. Posting it on Stack Exchange doesn't affect your rights to your own content.
If you copy any of the text from posts that were created by others, you must comply with the CC BY-SA license. The exact version will depend on when the content was posted, and can be viewed by clicking the "Share" link or viewing the post's timeline via the clock icon on the left. Currently, new posts are licensed under CC BY-SA 4.0, which requires you to (basically) provide attribution with the creator's name, a link back to the content, and an indication of whether changes were made. A more detailed description of the exact requirements is here. You would also be required to license the work that you incorporated it into under the same license.
However, game mechanics aren't copyrightable. If you merely used mechanics suggested in the posts without actually using the actual creative expression (for instance, names or description text) from the posts, you would not be required to provide any attribution or use any particular license, because you didn't use any copyrightable material from the post.
All that said, it's still a nice thing to do to provide some sort of informal thanks to those who provided valuable assistance, even when you're not legally required to do so.
User content on stack exchange sites is licensed under the CC BY-SA 4.0 licence. Appropriate credit is:
If supplied, you must provide the name of the creator and attribution parties, a copyright notice, a license notice, a disclaimer notice, and a link to the material.
Note that if you do include material (or create a derivative work) from stack exchange users, you must publish under the same CC BY-SA 4.0 license.
