dhava,
We're sorry to hear your contact doesn't have a device compatible with upgraded Reminders. It sounds like you're asking if you can revert back to the previous version of Reminders so that you can use the shared Reminders feature again with this specific contact. We can clarify further.
"Upgraded reminders aren't compatible with earlier versions of iOS and macOS. If you upgrade your reminders on your iPhone with iOS 13 or later, your other devices — like your iPad and Mac — can’t access the reminders until you update your iPad to iPadOS or your Mac to macOS 10.15 Catalina." With that said, once you've upgraded Reminders, you can't revert back to the old Reminders app. Have a look at the support article we posted above entitled, 'Upgrading the Reminders app in iOS 13 or later' to learn more about how that works.
"Shared lists require that both the creator of the list and the people who can view it have upgraded their reminders." Keeping that in mind, your contact will need access to a device running iOS 13 or macOS Catalina or later in order to use upgraded Reminders. To take a closer look at which devices are compatible with upgraded Reminders, view the links below:
iOS 13 Supported iPhone Models
iOS 14 Supported iPhone Models
iPadOS 13 Supported iPad Models
iPadOS 14 Supported iPad Models
macOS Catalina is compatible with these computers
macOS Big Sur is compatible with these computers
Take care.