I wish it could be $current_year. I was aware of this but decided to keep it as is for now. Skipping some years.
In case anyone is wondering, it’s not important for legal reasons to have this very up to date.
Issue is that mass updating this by a script (which I know how to do, which was done before) results in all packages being rebuild and a big package upgrade which does nothing except update the copyright year in the source files.
So maybe a release upgrade (rebasing to the next major version of Debian) is a good time to do this.