We talked to lots of CISOs, InfoSec managers and IT admins about that issue. There's basically two camps: Actively block any new tool vs. not block but educate so people don't do anything stupid.

I feel not blocking makes most sense. Employee's want to be treated like adults, especially in tech savvy companies. If they feel like they are unnecessarily blocked they will just find a workaround (i.e. non-work email or device).

However, you definitely want to keep track of people are signing up for - that's where the Shadow IT scanner comes in handy. In case you see something that's against policy it's often enough to just explain why it's a risk for the company. No employee means harm and just wants to be treated like an adult.