From Microsoft Corporation:
“We’re updating PowerApps custom forms integration with SharePoint libraries. To ensure the best experience we have updated the roll out timeline for this feature. We apologize for any inconvenience.
•We began gradually rolling this out to targeted release customers in November 2019, as announced in MC195725.
•We will begin rolling this out to all customers in early January 2020 (previously mid-November 2019).
•The roll out will be complete worldwide by the end of January 2020 (previously the end of November 2019).
This message is associated with Microsoft 365 Roadmap ID 57303.
How does this affect me?
PowerApps custom forms is already integrated into SharePoint lists, which provides the ability to create an app for a list directly from the SharePoint list experience.
We are now enabling this same sort of capability in SharePoint libraries.
What do I need to do to prepare for this change?
There is nothing you need to do to prepare for this change, but you may consider updating your user training and notifying your help desk.