"Copyright" is not found

on akismet.php, GPL license text is written. but “Copyright” is not written.

https://www.gnu.org/licenses/gpl-howto.en.html

Whichever license you plan to use, the process involves adding two elements to each source file of your program: a copyright notice (such as “Copyright 1999 Terry Jones”), and a statement of copying permission, saying that the program is distributed under the terms of the GNU General Public License (or the Lesser GPL).

copyright element is lack.

$ find wp-content/plugins/akismet -type f | xargs grep -i copyright | wc -l
0

https://www.ads-software.com/plugins/akismet/