IBM docs say .zsav is only possible in Unicode mode

[Albert-Jan Roskam]

The option to save the data file with your local code page encoding is not available for ZSAV files. These files are always saved in UTF-8 encoding.

Source: https://www.ibm.com/support/knowledgecenter/en/SSLVMB_22.0.0/com.ibm.spss.statistics.help/spss/base/savedatatypes.htm

So specifying ioUtf8=False when writing a .zsav should either raise a ValueError, or this should be ignored. In any case, the generated .zsav should be readable with SPSS Statistics.