"supportsUrlLoad" is the option that lets you add ?withgadget=gadgetname to the URL and load it that way. This is very useful for developers to do testing.

I am wondering why this is an option that has to be set in MediaWiki:Gadgets-definition, rather than just always true? Is there any downside to always having this turned on?

I am thinking we can set it to always true, and then delete the option, which would simplify the code, simplify the MediaWiki.org documentation page, and give developers a helpful tool out of the box.