Earlier versions of discord-bin (tested on 0.0.14) require CUPS to open, but v0.0.16 is completely functional without CUPS installed without any warnings when launched from the command line.
Had a look but libcups.so.2 is still needed with 0.0.16 $ objdump -p /opt/discord/Discord | grep libcups NEEDED libcups.so.2 $ discord discord: error while loading shared libraries: libcups.so.2: cannot open shared object file: No such file or directory If you were able to start it without then I assume either portage's preserve-libs mechanism kept the library or you have libcups available somewhere else.