I think I have it figured out, and yes, it seems we older version users are quite screwed.

I first changed just the port forwarding number to what had initially appeared in 4.16.3's preferences to see if my 5 connections would come back, but that didn't help. Then I remembered the extra connection stuff which appears in that version's Advanced>Performance area, so I decided to see what they would do in conjunction with my old port forwarding number. I changed it back and then unchecked the option to 'Disable Mojito DHT Capabilties.' It didn't help. But as soon as I unchecked the 'Disable TLS Capabilities'--I didn't even have to click 'Apply' or 'OK'--I saw the host listings appear in the Connections view and my 5 connections were made within seconds. (Previously I've had the GUI minimized while I was changing preferences so I couldn't see it happening.) Mind you I also have the option to connect immediately on startup disabled, so I don't understand how it could even begin to do that without my choosing the connect option from the File menu, but anyway...

Rechecking 'Disable Mojito DHT Capabilities' had no effect on my ability to immediately connect. So it's this secure communications mode that is wreaking havoc on people like me who aren't using a version equipped with the TLS capabilities. And it sucks.

I'm also seeing that the network is a bit less stable than it used to be, because I went through half a dozen drop-and-add connections in just three minutes, and that never used to happen to me so close to start up. (I used to see after a couple of hours that just two of my hosts had connect times which matched that, while the other three would be something along the lines of 30-45 minutes for two of them and 10-20 minutes for the last one.)