Mainline is not “unstable”
Note that stable does not mean more reliable or more bug‑free. In fact, the mainline is generally regarded as more reliable because we port all bug fixes to it, and not just critical fixes as for the stable branch. On the other hand, changes in the stable branch are very unlikely to affect third‑party modules. We don’t make the same commitment concerning the mainline, where new features can affect the operation of third‑party modules.
We recommend that in general you deploy the NGINX mainline branch at all times. The main reason to use the stable branch is that you are concerned about possible impacts of new features, such as incompatibility with third‑party modules or the inadvertent introduction of bugs in new features.
See