Gadgets that don't load via ResourceLoader require discrete requests to the server to fetch, which is awful for performance. Non-ResourceLoader loading of Gadgets should be sunsetted. The top priority should be fixing Gadgets that are turned on by default and which do not use ResourceLoader.
I'm in favor, but what about disallowing it first for gadgets that are enabled by default ?
A ton of gadgets stuff is only used by very few users, in those cases it probably matters significantly less. For default gadgets however, it is KEY to do this properly and it would probably achieve 80% of the same goal.
For many Gadgets, migrating to ResourceLoader will be as easy as adding [ResourceLoader]" to its definition on MediaWiki:Gadgets-definition.
What's the point of requiring an explicit [ResourceLoader] if gadgets can't work without it? Shouldn't it be implied now?