Hello! There is indeed in the sources extra/readline/.
Looking at upstream commit logs it has been there at least since 2014 (and traces of the same are in Oracle mysql sources too). This is not something new that has been introduced in 10.5, but rather very old legacy. The cmake/readline.cmake file should ensure the system libedit is always used in Debian. I will experiment by removing the whole extra/readline/ from the Debian sources and then file an issue about it upstream.
