Is there a page that shows how long Apple plans to maintain security updates for specific models? Something like what Google does for Chromebooks.
I am particularly interested in how long Apple plans to provide security updates for iPhone 6S Plus.
No.
The oldest devices supported by iOS13 released in 2019 are iPhone 6s (2015) and iPhone SE (2016), both with A9 processor.
The oldest devices supported by iOS12 released in 2018 were iPhone 5s (2013/A7) and iPhone 6 series (2014/A8).
If your iPhone 6s Plus (A9) is eligible to upgrade to iOS14 in 2020, I doubt it will be eligible to upgrade to iOS15 in 2021.
So you're basically telling me "IDK".
"No" is the answer to your question.
"IDK" is the answer to whether iPhone 6s will be able to upgrade to iOS14 and beyond. I gave an historical example so you get a sense that your device is likely nearing the tail-end for software support.
Apple maintains a web page with a list of vintage/obsolete devices, but this only indicates hardware not software support.
isn't there a pattern in the oldest supported devices? seems like they keep cutting off the oldest supported devices...
Thanks, this site helps!