Hi, Ken -
Here is a link to the supported Informix version information for PB 2017 R3:
https://docs.appeon.com/pb2017r3/connecting_to_your_database/ch09s01.html
Anyone can find this information; Open docs.appeon.com, select PowerBuilder, then expand the list of product documentation on the left side for the version you are interested in. The information you've requested is in the "Working with Native Database Interfaces" section of the "Connecting To Your Database" publication.
Most search engines can be used to search PB (and other Appeon) documentation, especially if you include "site:docs.appeon.com" (without the quotes, of course) in the search criteria. For example: Entering "site:docs.appeon.com powerbuilder 2017 r3 informix database versions" in Google brings up the same publication I've referenced above.
Regards, John
FYI: Also, PB2021 only supports the same Informix versions as PB2019R3 according to the PB Help. Please open a Support Ticket for this issue so that we can get Engineering involved.
Regards .. Chris