Windows 7. Some may say 8.1 if they were willing to tolerate it, but most will agree that Win7 was the last “true” OS that wasn’t riddled with adware and telemetry collection.
The same is happening with Windows 11 as well. People don't want to upgrade to it. With its pervasive adware and information gathering. The only reason the company I work for is buying Windows 11, is that you can't buy Windows 10 anymore.
Phones can have exponential growth in hardware capabilities, but the bigger thing is they don't need to get more powerful.
All the app makers want their apps to work on any device, so a 10yo budget phone with 1/60th the horsepower of a modern flagship will work just fine with most apps, and New phones are just way overpowered for what they actually do.
One another factor is hardware manufacturers started to screech at them to make Windows even cheaper for them, because Android is technically free (save for the driver licenses) in exchange for user data collection, so they started to kneel.