gerrit:197449 drops syntax highlighting for a bunch of languages on WMF wikis, e.g. lang="lotusscript". Yet there's no warning when you specify a dropped language, you just get a <pre> block with no highlighting. This sandbox revision demonstrates the glitch.
I think the issue is the extension's parserHook() doesn't check $wgGeSHiSupportedLanguages; it prepares a new GeSHi object and this successfully loads the lotusscript.php code despite the wiki not loading CSS to style the highlighting.
This may be moot if T93025: Combine infrequently-used language modules into a single module is addressed and/or the chechil fork of GeSHi unifies CSS. The extension's wiki page mentions this bug.