A bad SyntaxHighlight invocation like <source lang=html9> used to display an ugly warning which you could expand to a huge list of supported languages, which was good for editors but bad for readers:
Now it displays the unhighlighted source code without any warning, unless you notice the page receiving Category:Pages with syntax highlighting errors. That's good for readers, but no help for editors. gerrit 221311 adds messages for the warning, but has a comment "The actual warning text is not outputted anywhere yet; deferring that for a follow-up patch". The extension should show the warning on wikitext preview, along with its expandable list of supported languages (see related T103756); if the user saves the bad source tag, maybe SyntaxHighlight could show an inconspicuous error indicator, perhaps a ≸ superscript.