Add 3GPP TS 29.128 V15.6.0 (2019-09) Add Communication-Failure-Information, Grouped, code 4300, section 6.4.4 Add Cause-Type, Unsigned32, code 4301, section 6.4.5 Add S1AP-Cause, Unsigned32, code 4302, section 6.4.6 Add RANAP-Cause, Unsigned32, code 4303, section 6.4.7 Add BSSGP-Cause, Unsigned32, code 4309, section 6.4.8 Add GMM-Cause, Unsigned32, code 4304, section 6.4.9 Add SM-Cause, Unsigned32, code 4305, section 6.4.10 Add Number-Of-UE-Per-Location-Configuration, Grouped, code 4306, section 6.4.11 Add Number-Of-UE-Per-Location-Report, Grouped, code 4307, section 6.4.12 Add UE-Count, Unsigned32, code 4308, section 6.4.13 Add Connection-Action, Unsigned32, code 4314, section 6.4.18 Add Non-IP-Data, OctetString, code 4315, section 6.4.19 Add Serving-PLMN-Rate-Control, Grouped, code 4310, section 6.4.21 Add Uplink-Rate-Limit, Unsigned32, code 4311, section 6.4.23 Add Downlink-Rate-Limit, Unsigned32, code 4312, section 6.4.22 Add Extended-PCO, OctetString, code 4313, section 6.4.26 Add SCEF-Wait-Time, Time, code 4316, section 6.4.24 Add CMR-Flags, Unsigned32, code 4317, section 6.4.25 Add RRC-Cause-Counter, Grouped, code 4318, section 6.4.27 Add Counter-Value, Unsigned32, code 4319, section 6.4.28 Add RRC-Counter-Timestamp, Time, code 4320, section 6.4.29 Add TDA-Flags, Unsigned32, code 4321, section 6.4.31 Add Idle-Status-Indication, Grouped, code 4322, section 6.4.32 Add Idle-Status-Timestamp, Time, code 4323, section 6.4.33 Add Active-Time, Unsigned32, code 4324, section 6.4.34

The digested changelog can be found under:
 contrib/debian/changelog
 
You can also find the exhaustive list of change using mercurial history, 
or at the following URL: http://www.freediameter.net/hg/freeDiameter/shortlog