Neither Mordenkainens Tome of Foes nor Mordenkainen Presents Monsters of the Multiverse are canon. 5e 2014 never considered them canonical, and neither does 5e 2024. So it is incorrect to speak of these books as "default".
If a DM and players want to opt in and use the setting lore from these books for the setting they choose, obviously they can do that. Indeed, I never used MPMM during 2014, but am now cherry-picking thru it for 2024. But there is no default canonical pressure to do so.
The 2014 Players Handbook is no longer canon. 2024 replaces it, especially with regard to Elves. If players want to opt in to use the 2014 Half Elf they can. Or if theyd rather, play the Eberron setting and the Khoravar, or import the Khoravar into whatever setting. Or simply use the Players Handbook Human or Elf stats for the Half Elf. At the moment, there is no canon way to play a Half Elf.
Again, if players want to pull out their 1e setting books or 2e or 3e or 4e for the Elves there, they can do that. 2024 purposely makes it easy to do this.