Google didn't remove the functionality, it's still there but it doesn't provide interface anymore. It worked fine in the past so I can't imagine why they would shut it down. You still can add quick events but some work around needed. Go create a blank new form in Google Forms. Call it Google Calendar Quick Add and a single question/answer box mobile. Save it and generate public link (shortcut) and bookmark it etc. Now go to that form and create one new entry. It goes to Google Drive Google Sheets. Then go to Zapier to register free account if you don't have. Best create new account because free accounts are limited to 100 tasks per month, meaning success tasks. Create a flow from Google Forms pointing to your Google Sheet. There you have two options - Standard Calendar event entry and Quick Add Event entry. Go through the steps and you get a working version of Quick Add for Google Calendar. The beautiful thing is I have access to this anywhere I am, even a public computer. Actually I prefer use web-based Google Form for many apps over the native Android apps because Google Forms allows me to create simple application interfaces. I am running about a dozen "web-apps" using Google Forms as a front end.
What to use for automation:
To make things work I stick with Zapier because it offers decent integration (better than IFTTT) and it's a bit less complicated than Integromat. Zapier free account has limitation in 2 levels max. Integromat gives me unlimited levels (can knock myself out) but only 2 or "flows" per account.
Note, you can sometimes get an error in Zapier log (also error email message) saying "There was an error with Google Calendar: Required field "Describe Event" (text) is missing." That seems to be an issue with Google Drive access for Zapier but if it happens I resolve it by creating a new Google Forms answer sheet.
Hope this helps.