I'm just starting to use org-protocol, and I'm confused by this warning.
I can't find any description of the differences between the old and new
link styles in the documentation, or any pointers as to how and what I
should change to adopt the new syntax. The org manual links to
http://orgmode.org/worg/org-contrib/org-protocol.html, which seems to be
a year or more out of date.

I have shared a proposal to move org-protocol documentation to core documentation.
It’s mainly copied together from the old one and 9.0 NEWS.

The important part regarding the new syntax could be found here:

https://github.com/mgmart/org-mode/blob/documentation/org-protocol/contrib/orgmanual.org#L7347-L7364

Would that help or is there still something to add?

Kind regards
Mario