Are there any other sites that relate to YI hacking?

SMW Central is the main site with a forum for YI hacking discussion. There is also an older Yoshi's Island hacking wiki here.

Is there a Discord channel for YI hacking?

There is. Take a look at #misc-hacking on the SMW Central Discord.

I've created a Yoshi's Island hack (or a demo of one). How can I release it?

You can upload your hack to the Yoshi's Island Hacks section. Make sure you have read the guidelines. Use Floating IPS to create your BPS patch - do not upload a ROM to the section.

Alternatively, you can upload your patch to the My Files section of the site if you would like for it to be available to download and it does not meet the section guidelines.

Make sure you play through 100% of your created patch before releasing it.

You may also create a thread for your hack in Yoshi's Island Hacking forum, regardless of whether it's submitted to the hacks section, uploaded in the your files, hosted elsewhere, or even unreleased (as long as you have screenshots or videos to show). Make sure to tag your thread with the WIP Hack tag if your hack is unfinished, or the Hack Release tag if it is finished.