The 'company line' is basically that they provide full support (including Security updates) for the current version (now MacOS 15 Sequoia) and the Two previous versions (MacOS 14 Sonoma and MacOS 13 Ventura). Some major software vendors follow that same playbook, such a Microsoft Office (Word, Excel, PowerPoint, etc) products. if you depnd on those, they may become restricted at that pinch-point.
MacOS has historically been updated in the Fall of each year, so you should be prepared for 'no more security updates' after next Fall (northern hemisphere).
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Many newer features are being introduced as Apple-Silicon Macs only, including extended inter-operability with other Apple devices, 'Deskview' camera use in conferencing, and Apple Intelligence AI writing features. So there may not be a cliff you face in this case, but a constant siren-call of new capabilities, but "MY old Mac can not do that new stuff".
if you do not depend on Microsoft products or Major packages with a similar policy, and are not very concerned about new features and Security Updates, you can drive it till the wheels fall off. It may not be able to do MORE, but it can still do everything it could do when it was removed from the original box, and quite a bit more in addition based on free software updates so far.