Update: It appears that Magisk was not updated for a while on both phones, so the problem could be tied to an earlier Magisk update than just the very recent. Since I have access to TWRP on both phones, I guess I should flash an earlier version of Magisk from a zip, right? It seems that the version 1y back ran fine.
OK, one more update even though I am conversing with myself. However, someone may run into the post while trying to solve similar problems. Advice to go to device specific forums does not work for older less popular devices as such forums dry out with time. I managed to resurrect both Z2 Play phones that quit booting after the Magisk update. One got resurrected from a TWRP backup. Magisk will not be updated there from the version that worked. The other phone had no backup, but I managed to uninstall Magisk there using the Magisk-uninstaller-20210117.zip. Surprisingly just flashing the file removed Magisk. It is easy to run into instructions for uninstalling that are outdated and do not work, but this file did it for me. The experience of the near loss of the phone was so traumatic for the owner of that phone, that I was asked not to install Magisk ever again not just on the particular phone but any future one that the person might own.