Lynda and Peter,
Dealing with something very similar in my career for the last 40+ years and reviewing several Privacy rules here, on GEDMatch, and elsewhere, I believe that the GEDmatch ID number by itself, as long as there is no identifying information would not be a privacy violation issue. It's only when you combine the ID number with other personally identifiable information (such as true name, address, email, etc.) without the permission of that person, where it would become an issue.
I should be able to type in A138916 all day long without any problems. I would also say that this type of policy should also apply to an GEDmatch alias used as long as it's not tied to any other personally identifiable information.
Personally, I probably wouldn't use a GEDmatch ID number and an Alias at the same time because of all the people/lawyers out there waiting to sue somebody for a perceived breach of these new regulations.