Problem/Motivation
#3347291: Combine field storage and field instance forms changed the field/field storage form structure. In that issue, various calls to that trait method were updated to include the field_storage[subform] prefix, which is one thing for core, but it's quite tedious for contrib that still needs to support older core versions.
Paragraphs module tests have a _lot_ of those and I'm sure other modules use that trait as well.
Steps to reproduce
See #3376124: Prepare for 10.2: Move preconfigured paragrah field type to general category
Proposed resolution
Automatically prefix the storage keys so that calls do not have to be changed. This is a bit weird due to how array form keys work, I'm open for fancier implementations.
Remaining tasks
One thing that this doesn't cover is requests and posts directly to the /storage URL to change an existing field. As well as code that doesn't use the trait of course.
I've seen at least one example in paragraphs of the first case (editing existing storage settings). If there are more then I'll create a trait that we can share in a change record.