I have created a GitHub fork of MaxScale which does not include the 2.0 branch. The repository is called GPLScale.

There is a big potential problem with this.  Take for example the file server/core/utils.c, which now has a new license:
https://github.com/mariadb-corporation/MaxScale/blob/2.0/server/core/utils.c

This file WAS covered under GPL:
  https://github.com/mariadb-corporation/MaxScale/blob/cb3213af6382c380df9fc47347764f087ae7ecb4/server/core/utils.c

When there is a bugfix in 2.0 for server code that existed in 1.0, can that code be backported directly into GPLscale?

a) util.c in 2.0 is clearly derivative of 1.0, and is therefore a derivative work of 1.0 and therefore still covered by GPL and b) there is a "violation" of GPL, because 1.0 GPL code is being linked against code covered by an incompatible license (other code in MaxScale).

Almost all of the files in MaxScale 2.0 derive from 1.0 so this problem is not just in one file of course.

The FSF says that the owner of the software can't legally violate the GPL they can morally violate the GPL.