They have been distinct subraces since 1st edition D&D Forgotten Realms Campaign Set. And there are differences between the two subraces are significant within the Forgotten Realms setting.
In 3rd edition there were mechanical differences between the two (moon elves got +2 DEX, -2 CON, and sun elves got +2 INT, -2 CON)
The main differences between the two subraces are that the moon elves are the most common of the elves, the subrace most often found in human cities, as they posses a natural wanderlust and perhaps the most open-minded tendencies of the elven subraces. Whereas the sun elves are generally staunch traditionalists who cling strongly to the memory of their prestigious past (many of the great elven empires and nations of ages past were lead by sun elves). They often think of themselves as the custodians of elven culture and of the elven people in general, and consequently may sometimes come across as elitist or domineering towards the other elven subraces. Sun elves are also one of the elven subraces that are most rarely encountered in Faerun compared to moon elves and wood elves, as they generally dwell in elven lands like Evermeet or Evereska, or in cities with a particularly large elven population like Silverymoon.
I think, paying attention to details like this is important since this is Larian Studios' first dip into the Forgotten Realms universe, and ideally I would like them to depict it as faithfully as BioWare and Obsidian did
