There are a couple of candidates [0], the one i've seen the most speculation about is peganum harmala.

[0] https://en.m.wikipedia.org/wiki/Botanical_identity_of_soma%E...

Not sure, but I recently came across a book called Blue Tide: The Search for Soma by Mike Jay that you might find interesting if you want to learn more about the subject. Its on my list but I havent got to it yet.