Modern Horizons is removed from the legal set list and the additional rule about Basic lands has been rewritten.
Concerning Modern Horizons (MH1) ban. One of the Primordial format goals is to have a very large amount of playable cards. Thus, as MH1 is a set full of never printed before (nor after) cards, it seems to deserve its place in the format. Unfortunately, its power is such that it will not leave much room for more “standard” sets to express themselves and many cards or decks may never see the board because of it. Considering two solutions, either banning MH1 or banning a few cards from MH1, I finally decided to take the first path, at least temporarily. In other words, MH1 will make a comeback in Primordial (at least, I hope so) but it seems more relevant at this stage to let the format flesh out a bit more before its reintroduction.
Concerning the additional rule about Basic lands that has been rewritten. It went from “Any of the five non-snow covered basic lands can be played in any set” to “The following basic lands can be played in any set: 
Plains, 
Island, 
Swamp, 
Mountain and 
Forest“.