The WotC FAQ on the OGL (little alphabet soup there), which includes a link to the license itself.
Functionally, two cases:
1. If you directly host open game content (that is, people write whole games on the forum itself, or post big excerpts of mechanics and such), and if that counts as publishing, then the writers of same theoretically need to attach the OGL... Somehow? Nobody does this outside of SRD sites where they host a crapload of it, and nobody apparently cares, though some people do (rarely) link to it.
2. If you host self-contained publications that include open game content (that is, people put OGL systems and stuff up on Itch), then the writers of same need DO need to include the license in the publication. Everybody does this, and indications are that WotC does care, though not to any draconian extent.