I don't like it, FLAVORS are just changing the way something is built, it is not going to change what the software is.

Ok, let's take a step back.
Framework changes change the framework, they affect all ports.
When working on a framework update, it is the duty of the one working on the update to make sure all ports still work afterwards.
During the review process of framework changes, reviewers will often think of things you did not think about, and ask you to check stuff.
In this case, the easiest would be use distilator to figure out if a port that used to fetch does not fetch any more.
While I asked for you to use distillator to figure out if stuff would get broken, it was so that you would do the figuring out, I personnaly don't need to see the logs, diffs or such, they are means to help you figure out if stuff need to be fixed.

You probably need to sort the output of distillator, it is somewhat multi-thread and will output results as they come, which means its output is non reproducible, so, sort.

Can you test locally that all the ports using MASTER_SITES=GNOME still fetch with this change?

two small fixes.

with part two.

While the idea is interesting, the execution is quite unreadable, looking at the sed line, I have absolutely no clue of what it is doing.

Mmmm, 3.8 is the default, it should probably be at the start too.

Did you check that for each port in LEGAL, the port has a license that disallows dist-mirror?

fixup 3 ports.

rebase and move earlier.