I wrote an MSNP (Windows/MSN Messenger) client and used it for many years, all the way until Microsoft turned the protocol into a horrible XML-ised mess and then dropped support completely (as in, turned off the servers.) One standalone Win32 binary less than 32KB, usable from Win95 up to Win10; and over all the years I used it and noticed its memory usage, it was never more than 2-3MB. Yes, it had emoji support (though not very large ones, obviously.)

The amounts of memory that users are reporting for Slack are all several times higher than the entire RAM of the first computer I started using my MSNP client on.