Yes, names of files in
/etc/sources.list.d must end in
.list for lines in the files to be taken as specifying software sources that ought to be used. Otherwise, the files may be taken to have a different meaning.
man sources.list (1):
sources.list - Package resource list for APT
The package resource list is used to locate archives of the package
distribution system in use on the
system. At this time, this manual page documents only the packaging system used by the Debian GNU/Linux
system. This control file is /etc/apt/sources.list.
The source list is designed to support any number of active sources
and a variety of source media. The
file lists one source per line, with the most preferred source listed first. The format of each line
is: type uri args The first item, type determines the format for args. uri is a Universal Resource
Identifier (URI), which is a superset of the more specific and well-known Universal Resource Locator,
or URL. The rest of the line can be marked as a comment by using a #.
The /etc/apt/sources.list.d directory provides a way to add
sources.list entries in separate files. The
format is the same as for the regular sources.list file. File names need to end with .list and may only
contain letters (a-z and A-Z), digits (0-9), underscore (_), hyphen (-) and period (.) characters.
Otherwise APT will print a notice that it has ignored a file if the file doesn't match a pattern in the
Dir::Ignore-Files-Silently configuration list - in this case it will be silently ignored...
.list.save files are some kind of work/backup files: