Does imac 2019 usb-c port support Displayport alternare mode?

According to this it does, Apple iMac (Mid 2017) and newer, Apple iMac Pro, Apple Mac Mini (Late 2018) and newer.

Devices with Thunderbolt 3 and DisplayPort Alternate Mode over USB-C...

https://en.wikipedia.org/wiki/List_of_devices_with_video_output_over_USB-C#Desktops_2

https://www.monoprice.com/product?p_id=13234

If you press option-key + menu : System Information… does the displayed information for USB offer any status for displayPort Alternate Mode?

If you do not see it there, it is still not concrete evidence that it is not supported. As we are fellow users, and Apple engineering teams do not participate here, there would be no means to determine that alternate mode other than by your testing with compatible cabling.

Great to hear, good luck! :)