For example, many of the deities used throughout TSR's older work are directly or indirectly copied from mythology. I'm sure we could still use the names, etc, but what of the TSR - manufactured mechanics such as bonuses and granted powers of specialty priesthoods, and so on? Can we begin to include these in publicly available OGC on the premise that they're now "obsolete", or must we tweak them a little around the edges, claim the adaptation as your own invention and release it that way?
Mythology and anything over 50 years old is Public Domain. Those "places" and "people" can be used. However, anything that TSR used, such as the exact powers of Excalibur in Legends & Lore cannot be used. For example: King Arthur's Excalibur's scabbard was described as "While worn, the wearer cannot fall in battle." (paraphrasing. can't remember exact words used). D&D might have tacked on a few things or had AC at some ungodly number (don't feel like digging out my Deities and Demigods to find out). So long as your version of Excalibur isn't the same as D&D's, it's okay, because there was an Excalibur. Simple.
