Maybe with 10.16 coming later this year there will be another bootrom update and maybe these High capacity chips will work in older 2013 Imacs.

There may be more compatibility with the Haswell Cpu's, Don't know till we try?

It would be interesting to try the same chip 16gb DDR3L 1600 chip on a 2014 or mid 2015 Imac. One thing I haven't tried is running it with Win10, and at this point I won't since I've played with this enough and these chips are going back in the mail before I brick my wife's Imac, but if somebody were to run Win10 and see if it's a MacOs limitation or Hardware compatibility that would be another clue. At 51C, but that shouldn't be effecting the system. I checked the Dimm temp's and the 16gb chips do run a bit hotter than the 8gb chips. So at some point there is a degree of the CPU recognizing the Large ram chips but it's not able read from them entirely correct. I tried running it with 10.15.4 and it's still unstable.

Unfortunately today after a few hours I got Kernel panics with it in 10.14.6.

Max ram late 2013 imac