The Systems Reference Document (SRD) contains guidelines for publishing content under the Open-Gaming License (OGL). The Dungeon Masters Guild also provides self-publishing opportunities for individuals and groups.
Why does the SRD only have one background and one feat? Why do the PC races not include all of the subraces?
The goal of the SRD is to allow users to create new content, not to replicate the text of the whole game. We encourage players, DMs, and publishers to come up with their own backgrounds and feats.
Why is the SRD missing some spells, magic items, and monsters?
In general, the criteria for what went into the SRD is if it (1) was in the 3E SRD, (2) has an equivalent in 5th edition D&D, and (3) is vital to how a class, magic item, or monster works. For example, the 3E SRD has the delay poison spell, but in 5th edition that’s handled by the protection from poison spell, so protection from poison is in the SRD.