The dispute has reportedly been resolved; the Unions appear to have accepted that Guards will not be recruited:
A dispute that halted the much-anticipated introduction of new trains to Milton Keynes could soon come to an end, but not through the recruitment of…
www.thisisoxfordshire.co.uk
A dispute that halted the much-anticipated introduction of new trains to Milton Keynes could soon come to an end, but not through the recruitment of guards...
...While a second staff member will typically be present, train doors will still be operated by drivers, meaning the absence of a second staff member in rare cases will not prevent a service from running—mirroring practices commonly seen south of London.
The trains will be Driver Only Operated, but will usually have an additional member of staff present for checking tickets, customer service etc (i.e. similar to the OBS, OBM, TTI roles on DOO services operated by GTR Southern, Southeastern HS1 and Scotrail Strathclyde services respectively)
The article goes on to suggest that services wil be in the "December 2026 timetable", though it doesn't give a start date:
...In a written statement, rail minister Lord Peter Hendy said: "Chiltern worked with Network Rail, the Department for Transport and other operators on the December 2026 timetable and services have been timetabled between Oxford, Winslow, Bletchley and Milton Keynes."
It is great if common sense prevailed and the line can now finally open.