This showed up at the top of a tower today as the result of equipment replacement.
Does anyone have release notes?
Kind regards/ldv
Sorry, but we depend on the official page at http://www.mikrotik.com/download.html for release notes.
As MikroTik has said several times, this forum is not an official production of MikroTik.
As of this date, 3.31 is not an official release per said page.
kind regards/ldv
then this means that there is no such release? 
the following text should be treated as official MikroTik response:
v3.31 was released due to factory requirement for RB750 support. There are no changes in RouterOS features that would affect any other devices.
The company I work for has found the 4.x-series firmware to be too unreliable/unstable to trust on our access controller class devices and do not presently have the time to test the latest build.
Will RB750’s be shipping with 4.x firmware in the future? If so, could you please release the 3.31 firmware just for those devices on your web site so that we may downgrade if necessary?
Thanks!
Such claims have no weight and don’t help anyone unless you report your findings to support, so we can fix the problems you found. Unless you want to stay forever on an unsupported version