Do the itch.io webservers support it? I can't  find any info on that.  My JS file is at 7.2 MB now and I'm having doubts as to whether it's a good thing to have the user wait until it loads up.

Most web servers and browsers nowadays support on-the-fly compression of transmitted data. That's not the biggest problem. A lot of computers however, not to mention mobile devices, will struggle to load and run that much JS. You really need to consider reducing the scope of your game, or else making it native for people to download.