Safari 12 “Removed support for running legacy NPAPI plug-ins other than Adobe Flash” . Despite years of warning Cisco wasn’t quite ready (perhaps Apple has made a mess of the plug-in/extension migration )
… Apple have announced they’re removing support for NPAPI from Safari 12. This will affect the user experience for users accessing Citrix Receiver for Web using Safari on Mac. We’ll address this by turning on the Citrix Receiver Launcher for Safari 12+ in future releases of Citrix StoreFront…
With Safari 12 if you click on a Citrix Receiver link a .ica file is downloaded. You have to click the .ica file to launch Receiver. Prior to 12 the /Library/Internet plug-ins/CitrixICAClientPlugIn.plugin handled the .ica file, clicking a link caused CitrixICAClientPlugIn.plugin to launch Receiver. There’s a Safari 12 workaround, but I’ve not tried it.
Citrix does have new era extension support for Chrome, so you can just use Chrome until Citrix delivers a “Safari App Extension” version of the plug-in. (Which might come with their Citrix Workspace replacement for Receiver.)
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 The dev must have hated keeping Flash support. NPAPI is 1995 old, Chrome dropped NPAPI support in September 2015.
 Safari 12 also deprecated the newer-than-NPAPI “Safari Extensions” and Apple is shutting down the Extensions gallery. Instead we’re supposed to get Safari App Extensions, but, as is too often true of Apple, it’s not clear where one downloads Safari App Extensions.
 Installed in root Library rather than user Library.