Keeping myhacks support is a small price to pay for not breaking people's sites. Even if it is very very very few sites, breaking sites isn't something that should be encouraged. Even with 10 years of deprecation notices.
https://core.trac.wordpress.org/ticket/33741#comment:18 outlines all the ways that the hack_file and my-hacks options can be setup and thus all the ways that the removal of those options could break sites.
Fixes#33741.
Built from https://develop.svn.wordpress.org/trunk@35688
git-svn-id: http://core.svn.wordpress.org/trunk@35652 1a063a9b-81f0-0310-95a4-ce76da25c4cd