Remathilis
Legend
My guess.Since they're specifying that the '14 versions are still usable because nothing in the '24 PHB is "replacing" them, I wonder if that means the "Pick-a-Parent" system didn't make it in at all, or if it's just been allocated to a book further down the line (perhaps the DMG). In spite of my general grouching on this subject, I don't actually have anything against "Pick-a-Parent" in and of itself, I just feel that it's a partial solution at best and needs to be bolstered by something more robust.
Pick-a-parent will the the PHB "default" option for players/DMs who don't want to mess with the rules.
The 2014 options will be available to players who have access to older material.
WotC will probably create some system in a future supplement for people who want a meatier combination system (in a book where they can give that room and time).
The only players who would be locked out are ones whose DM's only will allow the 2024 PHB (and nothing else) and if that's your DM, you probably have bigger problems than half-elf access.