The only MQTT gateway on ESP8266 is done by Mickey who was commenting and has no pointer to his project. He has some links to a starting point in the comment.

Your first MQTT publish from an ESP8266 in three lines of code. With the addition of some callback functions, we can make it user-friendly and start receiving MQTT data as well.

Azure Event Grid’s protocol support now includes a public preview of MQTT, with support for MQTT 5 and MQTT 3.1 unveiled at BUILD 2023.MQTT support for both incoming and outgoing messages means ...