PowerApps and Flow are becoming a more integral part of SharePoint Online with the imminent release of Custom Forms with PowerApps and the Flow Launch Panel. Starting in November, these features will no longer be considered as preview features.
If you have the Preview Features switch turned off in SharePoint administration center today, you were not seeing the Flow and PowerApps buttons in modern lists and libraries. Once this change goes into effect, the buttons will become available, regardless of the setting: Flow button in modern libraries, and both Flow and PowerApps buttons in modern lists.
The change will start with our First Release tenants, and then move forward into the rest of the production in two waves. We hope to complete the change over the month of November.
PowerApps and Flow are still working on completing their certification for government and sovereign cloud environments with stricter compliance requirements, and the buttons will continue to remain invisible for these environments, independent of the preview features switch setting.
Source : techcommunity.microsoft.com