What version of Debian? In Bullseye, the version packaged is 2.22.
If you want to keep an older version of your distribution, you could
install 2.22 via the official generic packages from
http://lilypond.org/unix.html. These are pretty easy to use, just
unpack them and run the script
('sh lilypond-blabla.sh --prefix=/I/want/it/here' and I can't
remember if you need sudo; use --help for a list of options).
You don't (need sudo) if you're content to install LilyPond locally for your
user only. (Also you don't need to unpack them first: The files you download
_are_ the shell scripts.)
For example, I have a folder ~/lilypond-versions in my home directory. If a
new release comes out, I just do
mkdir ~/lilypond-versions/2.23.bla/
sh lilypond-blabla.sh --prefix /home/lukas/lilypond-versions/2.23.bla
And @Stefan: If you're using Frescobaldi, that program is perfectly equipped
to deal with multiple versions of LilyPond. Just add the newly installed
package in the LilyPond settings of Frescobaldi by telling Frescobaldi where
to find the "lilypond" binary file. (It's in the bin/ subfolder of the folder
you gave as --prefix.)
Lukas