Hi,
I don't believe there exists a roadmap for future releases, but there is a page that shows the lifecycle of the current Windows Server/Client releases:
Windows Server release information
https://learn.microsoft.com/en-us/windows-server/get-started/windows-server-release-info
Windows Client lifecycle fact sheet
https://support.microsoft.com/en-us/help/13853/windows-lifecycle-fact-sheet
For Windows 10, feature updates will be released twice annually with a target of March and September.
- March feature updates are serviced for 18 months from release date
- September feature updates are serviced for 30 months from release date
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Best regards,
Leon