Yaarel
🇮🇱 🇺🇦 He-Mage
Actually, the Elves continue to take on new forms. There will always be new kinds of Elves coming into existence.Monsters of the Multiverse mentions that the first elves originated in Arborea before migrating to the Feywild, and then later on to the many worlds of the Material Plane. Because of their original mutable nature, each group of elves mystically took on the characteristics of the environment with which they bonded to.
Forests- Wood Elves
Fey Crossings in the Material Plane- High Elves
The Underdark- Drow
The Shadowfell- Shadar-Kai
The Feywild- the Eladrin
The Oceans- Sea Elves
Originally, their mutability was at-will. Now the forms are thematic ambience, like Eladrin shifting appearances seasonally, and all Elves shift variously. They can still become new forms to adapt to a new environment, but it is moreso a community transformation, by implication via mythal.