Old versions of MacOS get about 2-3 years of additional security updates. So if you bought a 2017 laptop with a 7th gen processor, you got 8 years of support from 2017 to 2025. Unless you bought Surface Studio 2, which is $3499 from Microsoft and comes with a 7th gen chip so it only gets 4 years of support. If you bought a 2013 MacBook which just got cut off at Big Sur, you'll probably get supported to 2024, or 11 years.