As far as I can tell the regression has existed since the first import
for en_US which copied LC_COLLATE from en_DK (showing en_DK):
~~~
f5f52655ceb (Ulrich Drepper 1997-03-05 00:35:19 +0000 967) <A> <A>;<NONE>;<CAPITAL>;IGNORE
f5f52655ceb (Ulrich Drepper 1997-03-05 00:35:19 +0000 968) <a> <A>;<NONE>;<SMALL>;IGNORE
...
f5f52655ceb (Ulrich Drepper 1997-03-05 00:35:19 +0000 1546) <Z> <Z>;<NONE>;<CAPITAL>;IGNORE
f5f52655ceb (Ulrich Drepper 1997-03-05 00:35:19 +0000 1547) <z> <Z>;<NONE>;<SMALL>;IGNORE
f5f52655ceb (Ulrich Drepper 1997-03-05 00:35:19 +0000 1548) <Z'> <Z>;<ACUTE>;<CAPITAL>;IGNORE
~~~
Is this what you mean by "longer in the making?"