There has been 55 bugs fixed since 3.1.0.
Maybe the most notable is the one that caused gtk4-client to overwrite user-set options on every launch with ones migrated from gtk3.22-client. To provide that fix to users, 3.1.1 release should be made in a couple of weeks.
Upcoming 3.1.1
Re: Upcoming 3.1.1
What's fixed in 3.1.1: https://www.freeciv.org/wiki/NEWS-3.1.1