Formalize OS support policy (luanti-org#14556)

Author: sfan5

doc/developing/README.md

@@ -16,6 +16,7 @@ Notable pages:
 
 - [Developing minetestserver with Docker](docker.md)
 - [Android tips & tricks](android.md)
+- [OS/library compatibility policy](os-compatibility.md)
 - [Miscellaneous](misc.md)
 
 ## IRC

doc/developing/os-compatibility.md

@@ -0,0 +1,109 @@
+OS/library compatibility policy
+===============================
+
+This document describes how we decide which minimum versions of operating systems, C++ standards,
+libraries, build tools (CMake) or compilers Minetest requires.
+
+Most important is that we do not increase our minimum requirements without a reason or use case.
+A reason can be as simple as "cleaning up legacy support code", but it needs to exist.
+
+As most development happens on Linux the first measure is to check the version of the component in question on:
+* the oldest still-supported **Ubuntu** (End of Standard Support)
+* the oldest still-supported **Debian** (*not* LTS)
+* optional: the second newest **RHEL (derivative)**
+
+Generally this leads to versions about 5 years old and works as a reasonable result for BSDs and other platforms too.
+
+Needless to say that any new requirements need to work on our other platforms too, as listed below.
+
+### Windows
+
+We currently support Windows 8 or later.
+
+Despite requiring explicit support code in numerous places there doesn't seem to be a strong case
+for dropping older Windows versions. We will likely only do it once SDL2 does so.
+
+Note that we're constrained by the versions [vcpkg](https://vcpkg.io/en/packages) offers, for the MSVC build.
+
+### macOS
+
+We currently support macOS 10.14 (Mojave) or later.
+
+Since we do not have any macOS developer we can only do some shallow testing in CI.
+So this is subject to change basically whenever Github throws
+[a new version](https://github.com/actions/runner-images?tab=readme-ov-file#available-images) at us, or for other reasons.
+
+### Android
+
+We currently support Android 5.0 (API 21) or later.
+
+There's usually no reason to raise this unless the NDK drops older versions.
+
+*Note*: You can check the Google Play Console to see what our user base is running.
+
+## Other parts
+
+**Compilers**: gcc, clang and MSVC (exceptions exist)
+
+**OpenGL** is an entirely different beast, there is no formal consensus on changing the requirements
+and neither do we have an exact set of requirements.
+
+We still support OpenGL 1.4 without shaders (fixed-pipeline), which could be considered very unreasonable in 2024.
+OpenGL ES 2.0 is supported for the sake of mobile platforms.
+
+It has been [proposed](https://irc.minetest.net/minetest-dev/2022-08-18) moving to OpenGL 2.x or 3.0 with shaders required.
+
+General **system requirements** are not bounded either.
+Being able to play Minetest on a recent low-end phone is a reasonable target.
+
+## On totality
+
+These rules are not absolute and there can be exceptions.
+
+But consider how much trouble it would be to chase down a new version of a component on an old distro:
+* C++ standard library: probably impossible without breaking your system(?)
+* compiler: very annoying
+* CMake: somewhat annoying
+* some ordinary library: reasonably easy
+
+The rules can be seen more relaxed for optional dependencies, but remember to be reasonable.
+Sound is optional at build-time but nobody would call an engine build without sound complete.
+
+In general also consider:
+* Is the proposition important enough to warrant a new dependency?
+* Can we make it easier for users to build the library together with Minetest?
+* Maybe even vendor the library?
+* Or could the engine include a transparent fallback implementation?
+
+The SpatialIndex support is a good example for the latter. It is only used to speed up some (relatively unimportant)
+API feature, but there's no loss of functionality if you don't have it.
+
+## A concrete example
+
+(as of April 2024)
+
+```
+Situation: someone wants C++20 to use std::span
+
+MSVC supports it after some version, should be fine as long as it builds in CI
+gcc with libstdc++ 10 or later
+clang with libc++ 7 or later (note: no mainstream Linux distros use this)
+
+Debian 11 has libstdc++ 10
+Ubuntu 20.04 LTS has libstdc++ 9
+(optional) Rocky Linux 8 has libstdc++ 8
+Windows, Android and macOS are probably okay
+
+Verdict: not possible. maybe next year.
+
+Possible alternative: use a library that provides a polyfill for std::span
+```
+
+## Links
+
+* Ubuntu support table: https://wiki.ubuntu.com/Releases
+* Debian support table: https://wiki.debian.org/LTS
+* Release table of a RHEL derivative: https://en.wikipedia.org/wiki/AlmaLinux#Releases
+* Android API levels: https://apilevels.com/
+* C++ standard support information: https://en.cppreference.com/w/cpp/compiler_support
+* Distribution-independent package search: https://repology.org/ or https://pkgs.org/