This app was mentioned in 1 comment, with an average of 1.00 upvote
Android Apps do not need to spy on you. They are sandboxed, and if they do not get unlimited access to internet, then simply can't.
Most of my apps, do not have access to internet.
Even the Ad service (https://play.google.com/store/apps/details?id=com.zeropii.paperss&hl=en) does not have access to internet.
And if you the user install the internet gateway - which WILL be open source (https://play.google.com/store/apps/details?id=com.zeropii.firewall.service&hl=en) then you can block any access you don't want (try the demo - https://play.google.com/store/apps/details?id=com.zeropii.firewall.demo&hl=en)
With the IO gateway installed, then the Ads service get to access only one URL, that downloads data, and does not upload anything.
If we get full transparency at IO level, then you don't need everything open source.
Even more, many companies will simply not accept to make their software open source, but might be willing to provide transparency into IO.
And if you don't trust the IO gateway, that will be open source, and you can inspect the code, install it manually, and/or write your own one.