Hi, @Funnel !
Although you can no longer create private apps, existing private apps will continue to work.
If you have private apps on clients’ stores, please ensure that your contact information is up to date under Apps > Develop apps in the Emergency developer email field. This is how Shopify will get in touch with you if there’s a potential issue.
While there is no set deadline for when you might need to migrate a private app to a custom app, you can follow this guide to create custom apps if you’d like to make the switch now.