A lot of the long-term support for OSes are for security updates. I’m sure they haven’t added new features in a decade or more but security patches keep rolling out. These cover not just actual bugs in the code but also addressing evolving standards for encryption, hardware insecurities, etc.
https://www.cvedetails.com/vulnerability-list/vendor_id-26/product_id-17153/Microsoft-Windows-7.html
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