On Tue, 23 Mar 2021 16:37:13 +0200, you said:

I gave up for now and prepared bash script for removing the files, based on the information in https://www.cyberciti.biz/faq/debian-redhat-linux-delete-kernel-command/ (see "A note about custom compiled Linux kernel" section). In my opinion, if Makefile supports install, it should support uninstall as well.

Well, here's the thing.. the just-built kernel tree know what it has to install because it was just built with that .config, and it knows what files it just built. But how is that 5.12-rc4 build supposed to know what to remove that belonged to a 5.9 kernel built with a different .config that the 5.12-rc4 used, and which potentially had more modules enabled than the current source tree? Bonus points for knowing how to figure out if the 5.9 kernel was installed via rpm or dpkg or opkg or 'make install', and whether the 'make install' invoked the system-supplied /sbin/installkernel script, or the user provided a variant that installed more/less/different files? Especially as you noted, that it's a 3 or 4 line bash script to remove the files. And you hopefully know what extra rpm/dkpg stuff you need to clean up....