This is incorrect. Apple finished the transition to Intel on August 7, 2006 when it introduced the Mac Pro and an intel powered version of the xServe (remember xServe?!). The first version of OS X that required an Intel Mac was Snow Leopard, which was released in August of 2009, three years later. So based on history macOS won't drop support for Intel Macs until at least 2025, assuming the Intel to Apple silicon transition finishes two years from when it was announced at this summers WWDC.If the ARM transition is consistent with the PowerPC to Intel transition (and so far it's been dead on, including new CPUs in existing form factors), the last major update to MacOS that will support Intel will be next years. After that Intel Macs will receive at least three years of OS updates for their last OS release.
Beyond that Apple may continue supporting macOS on Intel longer if the installed user base remains high, it will depend on how quickly users transition to new hardware. Consider that iOS 14 still supports the iPhone 6s, which is a 5 year old device. Apple won't support Intel indefinitely, but its not going to pull the plug when a significant number of users are still dependent on Intel machines.