Ref this post from 2019:
How to purge the retained messages?
If you have mqtt issues with tasmota devices, you can effectively clear mqtt retain messages with tasmota device manager. Take a look at this https://github.com/jziolkowski/tdm
And if you have other devices and detailed info check out this http://www.steves-internet-guide.com/mqtt-retained-messages-example/
You purge a retained message by publishing an empty string to the topic as a retained message. It’s as simple as that.
You can do it directly from Developer tools > Services.
In the following screenshot, we are publishing an empty string (represented by two consecutive single quotes) to the topic your/topic/goes/here/ as a retained message (retain: true).
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This is pretty simple, but is there a quick way to bulk purge? I am pretty well looking at this situation now as a device has left 20 odd retained messages in MQTT.
Bit of a pain to purge them one at a time.
I have not found a way to bulk purge yet. Would be nice if there was a wild card to do it.
Use an MQTT client like MQTT Explorer. You still have to do it one topic at a time but the task is reduced to simply clicking an icon.
I can do it one topic at a time. Just wish there was a bulk way.
There is a bulk way by publishing to the group topic instead of each device
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What is a “group topic”?
I haven’t seen that term used outside of Tasmota.
Probably. I only use Tasmota.