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 deprecation does not affect hosted Mender, only on-premise installations of the Mender Server.
The reason for the deprecation is that the Mender client released with 1.7.0 and earlier does not properly handle HTTP2 connections and the on-premise backend will default to HTTP2 in Mender server release 2.7.0 and later. Newer versions of the Mender client handles HTTP2 correctly.
If you have any questions, please feel free to reach out via commercial support or the Mender Hub community forums.