Crestwave

You cannot fork the current project because it is not open source anymore. A fork of the last available GPL release would be possible, though.

Just because it’s open source

It’s not open source. The maintainer relicensed the project from GPL to the current source-available license last year.

The AUR package uses the last GPL release before the change and thus does the current license does not apply.

That is true, but bathing after avoiding it for 60 years is still a significant change in environment that probably isn’t great if you’re 94 years old.