It’s mentioned in this really good Verge article about SEO. I don’t think it’s a good sign for Reddit to just allow such blatant spam and makes me think how much subtler spam is out there too.
I’ve personally noticed more a few times in old threads you’ll find a comment made months later that’s recommending a product.
This is bad but the Fediverse is going to be just as, or even more, vulnerable to this kind of thing.
Fediverse is mostly open source. So anyone can implement anti-spam mechanism to their instance. Although it is currently open to spam, I think it will be much more spam-proof if it becomes mainstream.