The latest release version of this library is 0.1.8:

https://github.com/arduino-libraries/ArduinoMqttClient/releases/latest

Per the SemVer specification, the major version value of 0 indicates that the library is still in its initial development phase (AKA "beta"):

https://semver.org/#spec-item-4

Major version zero (0.y.z) is for initial development. Anything MAY change at any time. The public API SHOULD NOT be considered stable.

It seems problematic that in 2025, there is still not a "non-beta" version of an MQTT client for Arduino. This is a very common integration protocol for IoT and industrial IoT use cases, and should be a production-grade library fully supported by Arduino and the community. Similarly, any supporting libraries needed for reliable and robust MQTT communications (Wifi, TLS/SSL) should be production grade and supported by Arduino.