If you put data onto a networked device it may be sent to some place else.
If you don't want your data being shared:
Use a device that does not have any networking capability (both hardware and software wise)
Use a pen and paper, you can shred and destroy as you see fit.
If you're using an application on a mobile device with mobile data/wifi, the chances are, your data is being uploaded.
| I don't actually see this as a problem
Okay, go on, perhaps you have an interesting point
| and instead it's a PSA everyone needs to internalize
If it's not a problem, it's not a PSA because nobody needs to know or care. If it's something worthy of a PSA, then it must stem from a problem.
Having said that, you're right to be suspicious of commercial services, even that you pay for. Someone can found a startup with a strong commitment to customer privacy and the best of intentions, but a few acquisitions or near bankruptcies later, those commitments will go out the window.
The small chance that they might go out of their way to not sell premium users data doesn't seem worth much.
Further, a view that ignores many real world digital data risks faced by those considered to be useful targets; eg: compromised supply chains delivering "pre hacked" hardware with discreet wifi chips or hidden out of band comms, etc.