The "Z-Machine Standards Document"[1] has an appendix[2] with a very brief summary of the history of the Z-Machine; several of the other sections have tables (for the opcodes, and header formats, etc) with "V" or "Version" column showing when features were introduced.

[1] http://inform-fiction.org/zmachine/standards/z1point1/index....

[2] http://inform-fiction.org/zmachine/standards/z1point1/appd.h...