Answer: A new rule that applies to EVERYONE!
Weapon proficiencies as class features will work as normal. Martial Weapon Proficiency FEATS work differently in this game. If you take Martial Weapon Proficiency you get a weapon group as detailed in UA. Ex. You no longer get Martial Weapon Prof: Longsword as a single feat, you now get Martial Weapon Prof: Long Blades. Weapon Focus, Specialization, and Improved Crit (and other weapon-specific feats) work for all weapons in a group with which you are proficient. Exotic Weapon Proficiency feats still only grant a single exotic weapon.
New Weapon Group Proficiency: Valenar Blades: Double Scimitar, Scimitar, Kukri, and Falchion. Special: Only available to Elves of the Valenar region.
War domain for Keepers of the Past: Grants Martial Weapon Group Proficiency (Valenar Blades) and Weapon Focus (Valenar Blades).
This means: If you can't start with a double scimitar due to a lack of funds, start with two scimitars, a scimitar and shield, a falchion, a scimitar and a kukri, or whatever, and get the double scimitar later.
Also, taking Martial Weapon Group as a feat isn't distinctly inferior to an exotic weapon feat.