It seems that they have other plans for this beloved function of the mouse
The GNOME developers are preparing something that will get Linux users to reach for the pitchforks and take justice into their own hands.Not everyone was happy about the decision to remove right-clicking on the GNOME desktop, but people adapted by circumventing this “feature”.
The GNOME developers now plan to remove the middle-click as a paste option and replace it with something a lot less bourgeois. According to a commit on this matter, it will be used to “start selections, and provide text contextual menus (such as word definitions, sharing, etc.).”
The good news is that GNOME 3.10 will be left alone and user will still be able to enjoy pasting with middle-click. The modification will probably be made with the next development cycle for GNOME 3.12. Whether this is a good thing or not, it's up to you to decide.