Choices column is now supported in Portal and is GA in 2021 Release Wave 2 from Feb 15, 2022:
However, you may find it not working, and there is no official document about how to enable it.
The solution is to create a Site Setting record to enable the feature:
Name: Control/EnableChoices
Value: true

And that’s it! It should show up on Portal like this:

To learn more ways to configure it on your Basic Form / Advanced Form / List view, visit here:
https://docs.microsoft.com/en-us/powerapps/maker/portals/configure/choices-column
Cheers!