What happened to Network Diagnostics?

I've been working in MacOS 10.12 Sierra until recently. My ISP (Comcast) tends to be unreliable, and I've found the Network Diagnostics utility useful when a Web page won't load. The list on the left side of the window (below) shows where the problem is. I put an alias of Network Diagnostics in my XMenu so I can open it quickly whenever it's needed.

But now I discover that Network Diagnostics is missing in later MacOS versions (Mojave and Catalina anyway, haven't looked at High Sierra). And I see that the Network panel in System Preferences no longer has the [Assist Me…] button that opens the Network Diagnostics utility.

There doesn't seem to be any other way to get this information quickly and simply. There's a Wireless Diagnostics utility, but it takes time to generate a lengthy, complex report, rather than simply showing what's wrong where in real time. In any case, WiFi is almost never the problem.

I tried copying the Network Diagnostics utility to Catalina, but it wouldn't open; looks like some necessary library is missing.

A search for "Network Diagnostics" in Apple Support finds nothing; apparently it's been unpersoned.

Why has this useful utility/function been removed? Or has it been moved to some other location, and if so, where?