So, this is a problem with using IRL terminology with fantasy creatures. IRL, even "species" is somewhat loosely defined, and there are plenty of disputes over whether related animals are the same species or not. Subspecies is an even less well defined term that usually describes a geographically and to some degree taxonomically distinct group within a species. They are still capable of interbreeding but with significantly reduced viability. Basically, the subspecies is well on the way to being a distinct species.
But in D&D and a lot of other fantasy, subspecies sometimes seems to denote mostly cultural distinctions, or historical distinctions, or sometimes distinct abilities, or sometimes even distinct origins. It's a mess, really, which is particularly problematic given that subspecies is already a messy term IRL.
So, what is racist? These are fantasy races/species, so you can't really be a racist towards wood elves or orcs or something. However, it can have IRL racist connotations if the the fantasy species is depicted in a way that evokes IRL racist tropes (i.e. black=bad, etc.). It is also problematic, IMO, if fantasy species are depicted in a way that emulates IRL racist thinking, even if no particular IRL racist trope is used. This is why I have a problem with stereotyping, so that all, or virtually all, members of a fantasy species are treated as basically identical (i.e. all dwarves are skilled miners who covet gold, dislike elves, speak "Dwarf," wield hammers or axes, wear heavy armour, and have Scottish accents). And that sets aside them all sharing an alignment, but I think alignment is a ridiculous concept at all levels and don't bother with it. But, aside from dubious connotations, I also think fantasy stereotyping is super boring and unimaginative, and ultimately mostly amounts to copying Tolkien, badly.
So, to the OP, I think "subspecies" is such a vague concept that it could mean almost anything, but as long as it just denotes some minor physiological differences, likely as the result of extended geographical isolation, then I don't think it's a problem. But relying on stereotypes is lazy writing so stay away from it.