Comment 5 for bug 1983091

The postinst script already has a mechanism to fix deprecated options from the conf file:

https://git.launchpad.net/ubuntu/+source/mysql-5.7/tree/debian/mysql-server-5.7.postinst?h=ubuntu/bionic-devel#n112

# Default config in 5.5 and older 5.6 installations contains two deprecated
# options that were removed in 5.7. They were renamed, so we can fix this
# automatically. We create a backup of the old file

---

More upstream doc pointers on the removal of those options:

https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_myisam_repair_threads

https://dev.mysql.com/doc/refman/5.7/en/myisamchk-repair-options.html#option_myisamchk_parallel-recover