Yealink - EoL Products links to End of Life Announcement for end-of-life phones. Yealink seems to provide no list of when products will reach EOL.
Yealink Support - EOL Products
Yealink - Yealink Forums » IP Phone Series » T4x Series » Current Firmware T49G? 51.80.0.100 is older than 51.25.0.15.
Yealink Forum - VPN and SHA256 hashes: “W52P with firmware 25.73.0.20 does not seem to support certificates signed with SHA256.” (2014-10-10)
Yealink Forum - openvpn sha256 support: “sha256 will be supported in V80 with phone model T19_E2/T21_E2/T23/T27/T29/T4x” (2015-06-06)
Yealink Forum - t20p and sha256 for SIP TLS “T20 only supports sha128, currently there is no a plan to add the support of sha256 in this phone model.” (2015-08-27)