As part of this evolution of Microsoft 365 solutions we will be retiring the SharePoint Add-In extensibility model and believe Microsoft 365 customers will be better served by modern SharePoint extensibility models. SharePoint hosted Add-Ins primarily bring UX elements to the site the Add-In was installed into and the app web that was created for the Add-In. A common use case is showing SharePoint Add-In web parts on pages of the host web, using SharePoint Framework based web parts is the recommended alternative here. Provider hosted SharePoint Add-Ins run outside of SharePoint and typically have their own UX. They use Azure ACS as auth layer to enable calling back into the host web where the SharePoint Add-In was installed. The auth model for these will be based on Microsoft Entra ID. If there are requirements to integrate in the SharePoint UX then SharePoint Framework can be used as calling into external application APIs can be done from SharePoint Framework solutions. The retirement of SharePoint Add-Ins applies to Project Online as well, any SharePoint Add-In used in Project Online will follow the same retirement path as SharePoint Add-Ins used in SharePoint Online. To understand if your organization is using SharePoint Add-Ins or begin planning migration to SharePoint Framework based solutions or alternatives, we recommend that customers run the Microsoft 365 Assessment tool to scan their tenants for SharePoint Add-In usage. SharePoint hosted Add-Ins can be replaced by SharePoint Framework web parts, for the provider hosted Add-Ins any software as a service application platform in combination with Microsoft Entra ID for auth can be used as alternative development model.
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Read more at https://bit.ly/3sTQmGF
Source: Microsoft Tech Community SharePoint Blog
