The process of building custom applications and tools that interact with Microsoft SharePoint, including SharePoint Online in Microsoft 365.
Your existing site will not become entirely read-only in March 2027 as a result of this change. For your specific situation, the key dates are:
- Starting March 1, 2027, it will no longer be possible to create new classic publishing sites, including sites based on the BLANKINTERNET#0 template, or activate the classic publishing features.
- Starting October 1, 2028, user-created classic pages in existing tenants, including publishing pages, will remain available for viewing but can no longer be edited. Users will also no longer be able to create new classic pages.
For tenants created on or after March 1, 2027, the restrictions on creating and editing classic pages will apply from that date.
This change does not make the entire site or all its content read-only. List and library views, as well as list forms, are not included in the current scope of this change.
Regarding the statement you quoted, it means that using modern pages within an existing publishing portal based on the BLANKINTERNET#0 template is not a supported modernization approach. Therefore, removing the custom master page and recreating the content as modern pages within the same site should not be considered a supported way to modernize the portal. The Microsoft guidance you referenced recommends creating a new Communication Site for this scenario.
To keep the required content editable after October 1, 2028, I recommend planning a gradual transition to modern pages in a new Communication Site. You can begin with a few representative pages and verify that the content, links, permissions, and custom functionality work correctly before migrating the remaining pages.
Customizations should be evaluated separately because not everything can be converted automatically.
For more information, refer to the: Modernize classic publishing portals guidance,
Hopefully, this helps clear things up and clarifies which modernization path is suitable for you.