In our agency we work with lots of different clients, who usually don’t work with design software. Commenting on prototypes directly helps our workflow very much, but I can’t expect all of our clients to sign up for a sketch account.
Have you considered making a setting to let guests comment on designs in the Web App?
It’s interesting that Penpot had the same request from users and they explained a several problems related with unregistered commenting (that includes security ones and spam among others). I’m looking forward for Sketch’s team thoughts on this subject.