What will be the minimum Client version for 3.1 servers?
The client 3.1.6 should be a no-brainer in my opinion, because as of this version user are able to connect via the new myTeamspeak Nickname feature, one of the biggest user experience feature update for TS3 in the last years.
So I really hope 3.1.6 will make the cut as minimum required version to join the new servers.
Would make little to no sense for me to not force it, because then you still can't advise the Nickname for your TS3 server freely, because some user will get errors when they try to connect via a Nickname in a old client and wont understand it, since thier friends can connect just fine.