I am not aware of significant use of 31 bit mode.

But I don't see the benefit of annoying and discouraging users who want to experiment with Python and with Linux on Z in 31 bit mode.  Yes, maintenance theoretically is a burden, but there have been no recent issues that were specific to Linux on Z in 31 bit mode.  In fact, most of the original Linux on Z support issues that I opened were endianness issues, which aren't ameliorated by removing 31 bit support.  As others have expressed, deprecating 31 bit mode only removes the configuration option with no other code simplification.

It seems that it would be better to leave the configuration alone until there actually was an unresolved issue that motivated removal.  But I am not aware of any client requirement to continue the support.  Leaving the 31 bit configuration unchanged is more about maintaining good will among the volunteers who are interested in the platform.