Yes, mosquitto v3.1.1 is running fine.
I haven’t had chance to implement it into emoncms yet, but as far as mosquitto is concerned, it’s running together with mosquitto_client, on my stretch installation OK.
I had no unmet dependencies or errors in the install logs.
The best way to install appears to be …
sudo apt update; sudo apt upgrade -y
sudo apt install -y mosquitto mosquitto-clients
Paul