Since T75810, gadgets are not loaded at all when they're not defined as using ResourceLoader. Given there is no possible worse outcome, they should instead use ResourceLoader by default, as if the ResourceLoader option had been specified. Then users can identify issues with any.

Performing hundreds such edits doesn't strike me as a particularly useful use of user/sysop/dev time. We can spare the effort on the several hundreds wikis which have not been so fast as to update their gadget definitions yet.

The original question, for which an answer could not be found:

In T75810#1549927, @Nemo_bis wrote:

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?