I know Solasta is using the dnd 5.1 SRD ruleset. Even though not part of the Dungeon and Dragons universe, the game is still steep in the dnd 5e rules.

Realms Beyond says they are going to use the 3.5e SRD ruleset.
Quote
"Turn-based combat, featuring many favorite spells, feats and actions, based on rules described in the (3.5e) Revised System Reference Document (SRD) covered by the Open Game License v1.0a (OGL) by Wizards of the Coast, Inc"