Interbreeding and mixed-race children among the Wisdom Species were not uncommon, especially in areas where wars and famines had severely reduced the numbers of able-bodied men. Such hybrid children were often found among half-beastmen and halfling populations. However, half-elves were extremely rare.
Not just rare, they were almost non-existent.
Putting aside the scarcity and distance from human society of elves, and the discrimination and extermination policies humans had towards them, even if an elven mother was fortunate enough to give birth to a half-elven child, the immediate family's first action would be to kill the infant—usually being thrown to their death, in some regions suffocated or strangled. Practices may have varied by region, but the results were the same.