Target Display Mode from MacBook Pro to iMac (the version they don't support)

I am wondering if there is a way to make my iMac the second display for my MacBook Pro? I understand that my iMac (Retina 5K, 27-inch, Late 2015) is not supported, but wondered if I could kick it off the other way around? Or if Apple will ever support it. I have the thunderbolt cable and the adapter.

MacBook Pro (15-inch, 2018) still on Mojave - and iMac is now updated to Catalina.