I suggest to drop all @euro locales because they are currently not different than their non-@euro counterparts. I guess historically it was needed to have two versions of some locales when euro currency has been introduced but the previous currencies were still in use. Currently all old currencies have been dropped completely and replaced by euro even in non-@euro locales.
On the other hand, those @euro locales serve one purpose: the default code page for ex. de_DE is ISO-8859-1 while for de_DE@euro the default code page is ISO-8859-15. (Note that most of the current systems use UTF-8 so the old 8-bit encodings have limited use.) So probably what actually should be done is:
* drop the *@euro source files,
* drop the @euro locales from SUPPORTED,
* add ISO-8859-15 encodings to SUPPORTED (this mostly will mean rename "@euro" to ".ISO-8859-15"),
* add aliases: *@euro to *.ISO-8859-15 so the systems which refer to the @euro locales still work and do not need to switch to ISO-8859-15 explicitly.
Note that in ISO-8859-1 locales the "€" symbol is transliterated to "EUR".
Sounds reasonable to me. If you can predict what happens with Brexit we can make a mint on currency speculation. cjl