What differences are introduced between ICAO Annex 10 volume IV amendment 85 and amendment 89?
With help of Gerry and Mins, it became clear that Amdts 86,87 and 88 did not introduce any changes to Volume IV of Annex 10.

Therefore any differences between Annex 10 Vol. IV Amdt 85 and Amdt 89 would be introduced by Amendment 89. From this documentPDF it can be seen that the changes where distributed in state letter 2014/21.

Digging a bit further I found the 
minutes of an ICAO Communication Navigation and Surveillance  Sub-Group (CNS SG)PDF  meeting which included the state letter in Appendix B. You can find it from page 56 to 91. 

Amendment 89 contains over 30 pages of changes to Volume IV of Annex 10 and that was more than I had hoped for; needless to say I am not copying all that information into this answer. A summary of changes can be found below.




General changes


Various recommendations and clarifying notes added. 
Various editorial changes

Chapter 3, Mode S Surveillance


Revoking of the Mode A/C/S all-call interrogations after 1 January
2020.
Introduction of new data parity overlay ("Modified AA") in
transponder and Overlay Control (OVC) bit in Mode S uplink protocol.
Changes to Type Control (TCS)

Chapter 4. Airborne Collision Avoidance System


No significant changes

Chapter 5. Extended Squitter (ADS-B)


Introduction of ADS-B 1090ES Version 2

Chapter 6. Multilateration


Spectrum protection requirements