In the old map viewer there was a way to generate short links that was moved to the details page of the web map if you are using the new map viewer. Now it's gone from there too. What happened?
https://doc.arcgis.com/en/arcgis-online/manage-data/item-details.htm
The shortened URL is still there in the Share menu of the classic map viewer.
You also note that this function is in the Details page of a given item (it is still there for me). This, however, generates a link to the Details page, not the map viewer itself.
When you open the sharing menu or click the sharing button on the Details page, what your browser is doing is sending a GET request to https://arcg.is/prod/shorten with the url parameter longUrl=<your url here>.
While I do not know why that particular function is missing from the new map viewer, it's worth noting that you can use the URL shortener with anything. For instance, here's a shortened URL to this very thread: https://arcg.is/0v80P90
With a bit of fiddling, you could easily create a script or tool that auto-generates shortened sharing links for maps yourself. But I would check the Ideas board to see if anyone has suggested this be added to the New Map Viewer, and suggest it yourself if nobody has yet. I'd vote for it!
Thank you Josh! You are correct that it's still part of the old map viewer, and that it takes you to the details page from where you can launch the map itself.
What I was referencing is that it is no longer on the details page of web maps where I'm using the new map viewer (or the details page of any web map for that matter). Maybe its my browser or something else, but I'm not the only one in my org experiencing the issue at the moment and thought it might be an ESRI issue, but maybe it's my org?