It's D&D 5.5e, akin to what 3.5 were to 3rd edition. I think they kinda dropped the "One D&D" title IIRC.
It seems they are aiming to release it for the 50th anniversary of D&D and it will have quite an amount of tweaks or straight up reworks of classes and mechanics. On top of my head: floating racial bonuses, all classes getting their subclass at third level, weapons getting properties akin to what has been introduced in BG3, rework of the exhaustion mechanic, and so on. Way too many to list them here.
