Check out the MSDN documentation:
Find the Report Definition Schema Version - SQL Server Reporting Services (SSRS) | Microsoft Docs[
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How to: Identify the RDL schema version of a report
Open the report .rdl file in an application such as Notepad or XML Notepad, in which you can view the XML.
The XML Report element specifies the schema namespace. For example, the following Report element specifies the namespace for Report Designer and the namespace for the report definition.
XML
<Report xmlns:rd="http://schemas.microsoft.com/SQLServer/reporting/reportdesigner" xmlns="http://schemas.microsoft.com/sqlserver/reporting/2016/01/reportdefinition" xmlns:df="http://schemas.microsoft.com/sqlserver/reporting/2016/01/reportdefinition/defaultfontfamily" MustUnderstand="df">
The most recent report definition namespace is 2016. However, the most recent published report definition namespace is 2010, specified by the following URL: https://schemas.microsoft.com/sqlserver/reporting/2010/01/reportdefinition
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More details you'll find under above link.