> Most of this opinion comes from “SQL” being vendor-specific. Is JSON vendor-specific? Is anything else, that we actually use by choice?

Two things that come to mind are Markdown with all its flavors and Regex with multiple engines.

edit: and to a lesser extent maybe C/C++ compilers and JS engines.

edit2: also JVM, Python and Ruby runtimes

But both edits describe technologies with an official spec and slightly different implementations. Markdown/Regex are more comparable to SQL because they have vendor-specific syntax.