Great suggestion. I've installed the plugin and it is mostly working.
Right now the only problem I have is the need to refresh the Web UI in order to get the red banner when a connection failed. It takes about 15 seconds before the banner appears if you constantly refresh.
I tried two browsers and ...

Thanks for the help.
Right now, it seems like the sending a "probe" message with a script is probably the most straightforward approach -- it's a good suggestion.
I've also posted a question on the Mosquitto forum to see if, by chance, they log any connection failure which could be put on a "bad ...

Thanks for looking further into this, those references are great.
Everything makes sense now. I'll have to think about the best way to proceed.

--Len

Thanks to all for quick replies.
I do understand that I could modify the Tasmota code and add/or add a script in Domoticz to get behavior close to what I want.
However, I'm a bit perplexed that the behavior I want isn't more directly available for the following reason:

It seems like MQTT uses TCP ...

Configuration:
Domoticz on RPI 4 running RPI OS.
Sonoff S31 plug flashed with Tasmota firmware
Sonoff is plugged into mains and a lamp plugged into the Sonoff
Two Hardware items:
Dummy for switches
Mosquitto – MQTT Client Gateway with Lan Interface
Virutal switch (on/off) for Sonoff shows ...