westes
Update for COPYING (last date 2007)?
You're quite correct that COPYING and any other copyright notices need updated. I have a vague memory that some automated tool exists to do this, though.
Yes, there are automated tools, for example gnulib's update-copyright or Bison's update-package-copyright-year, but those cannot be post-applied. From my understanding the years should be updated when copyrightable changes were done, COPYING is a common file so we can just inspect the commit logs if every year has changes, then change that to -2021.
The same can be done for the library version (if that is printed/returned somewhere) and the output of flex --version.
For other copyright notices I'd say that should be investigated per file (again with the help of the git logs), a possibly good option is to run LANG=en git log --follow $file | grep Date - but the "per file" part could also happen after a release (because the last one hadn't those cleaned up either).