Mender 2.6 will be the last on-premise release series to support Mender clients 1.7.0 and older out of the box. It will still be possible to support these clients by performing extra configuration steps, and more details about this will be posted in the announcement for the Mender 2.7 release. However, the recommendation is to upgrade the client to 1.7.1 or newer before upgrading the server to 2.7, so it is time to start planning this now.
This is a companion discussion topic for the original entry at https://mender.io/blog/deprecating-mender-1-7-0-and-older-on-premise