I've believed this since 1994 - that if you really want to turn off a cell phone, you need to take the battery out.
But why? What is a good technical reason to keep spending battery charge and occupying tower bandwidth despite the user having shut down?
Please, if you could refer to sources, alongside your provided reasons, I would love it very much!
Clarifications: I am aware of locking your screen, which (depending on your settings) can make the screen turn off. My question is clearly about shut down. Not locking screen, not screen locking itself after a minute due to inactivity... I am talking about pressing the power button and choosing "shut down" (Nokia) or "Power Off" (Android)
It's very hard to find discussion about this topic because of millions of people raising all sorts of power issues on their phones but I really don't know of anyone who doesn't believe the baseband processor remains on even when you shutdown the device, including experts in the field.
For example, here's a guy who appears to know a thing or two about Android phones (look at the most popular answer to the question): https://android.stackexchange.com/questions/215736/what-is-the-fastest-way-to-shutdown-unrooted-android-phone
Another example: https://www.zdnet.com/article/fbi-taps-cell-phone-mic-as-eavesdropping-tool/ "Some handsets can't be fully powered down without removing the battery" ...Again, I know these aren't design documents by Qualcomm but people seem to be adamant that it's a thing! Would be lovely if they cited their sources :D