Is now enabled on signup and can be edited at any time from your profile. To set it right now go to: https://webflake.sx/settings/ and hit the "Edit Profile" button on the right. (Seriously ips, why is this button so small and also not in the user dropdown?)
You can only use numbers and dots in this field. My suggestion is to leave the minor version (last digit and dot) out. So for example 4.1.13 is enough.
The reason this field was added is to possibly speed up support requests. It's not required so if you have legit security concerns you can leave it empty.