I don't imagine this was a 100% their decision, it's more like a response to the epidemic of all the world's governments suddenly coming up with adult verification schemes. Discord has already required it in some countries, and it's definitely easier to get everybody to verify themselves than require it on a per-jurisdiction basis. The personal data they get is a cherry on top.
Also, this is just the beginning, more social networks will require the same soon.
I wouldn't call it a response, but rather another thing that normalises ID verification online. Now all these governments can use Discord as a reference that (1) this is possible at scale and (2) companies are willing to do this.
Also, this is just the beginning, more social networks will require the same soon.