thank you, john. apologies for this. unbeknownst to me, anaconda python had installed this old version and overwritten my path. so, here it is again, now with a 2.6 version:
# pandoc --version
pandoc 2.6
Compiled with pandoc-types 1.17.5.4, texmath 0.11.2, skylighting 0.7.5
Default user data directory: /Users/ivo/.pandoc
Copyright (C) 2006-2019 John MacFarlane
Web: http://pandoc.org
This is free software; see the source for copying conditions.
There is no warranty, not even for merchantability or fitness
for a particular purpose.
# pandoc -o output.md forpandoc.pptx
Unknown reader: pptx
# pandoc -o output.md forpandoc.ppt
UTF-8 decoding error in forpandoc.ppt at byte offset 0 (d0).
The input must be a UTF-8 encoded text.
# pandoc -o output.md forpandoc.odp
UTF-8 decoding error in forpandoc.odp at byte offset 16 (ac).
The input must be a UTF-8 encoded text.
the ppt, odp, and pptx were all written by the same powerpoint program from the same input file.