It's worth bearing in mind that the Safari major version and OS version are tied together.
Latest Safari cannot be used by users who cannot run the latest MacOS on their hardware, or have reasons not to (such as bricking risk with latest MacOS on old hardware.)
(I imagine it's similar with iOS but I'm not sure.)
You have it backwards, there is no separate installation of Safari for iOS or iPadOS, it only comes as part of an OS upgrade. Major (named) macOS updates come with a new version of Safari but it is also available to install on the two previous major versions of macOS. The current version, macOS Monterey (v12), came with Safari 15 and Safari 15 is also available for macOS Big Sur (v11) and macOS Catalina (v10.15).
The iPhone 6s was released in 2015 yet can run the latest version of iOS released in 2021. It probably won't be able to run the 2022 version of iOS but at that point it will have been seven years. I'm sure Android can't be updated for that long, I don't know if a current browser for Android can be run on a seven year old version of Android.
The latest version of Firefox (97) at least claims to run on Android 5.0 which is from 2014. It runs fine on my Retroid Pocket which is on Android 6. So at the very least that's available.
This is not correct, you can update to the latest version of Safari on all supported versions of macOS. Currently, Safari 15 is available all the way back to macOS Catalina (from 2019...)
This is part of the problem with the world of computing today. When a software released not quite 2 years and 4 months ago is referred to with the phrase "all the way back". Don't get me wrong, I'm all for keeping things as up-to-date as is reasonable, but the industry has far too much churn in many areas.
2019 is _ancient_ in terms of web browsers... What you call churn is actually mostly moving the web platform forward and fixing exploits. There's no way for the modern web to work without the rapidity of development browsers are undergoing.
Pretty sure that's the OS we're talking about, macOS Catalina 10.15? I am running it right now, and not pining for the new shininess of the more recent OS releases. If it wasn't still getting security updates, sure (but I've got a few more months, at least).
For MacOS this is not true. You will get Safari stable decoupled from the operating system going back two versions (Catalina and Big Sur). What you cannot run is Safari Technology Preview, that is available only for the current OS version.
Latest Safari cannot be used by users who cannot run the latest MacOS on their hardware, or have reasons not to (such as bricking risk with latest MacOS on old hardware.)
(I imagine it's similar with iOS but I'm not sure.)