Proposal: deprecate and remove feature-exists (#3749)

proposal/remove-feature-exists.md

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+# `feature-exists` removal: Draft 1.0
+
+*([Issue](https://github.com/sass/sass/issues/3702))*
+
+This proposal removes `feature-exists` in the built-in `sass:meta` module.
+
+## Table of Contents
+
+* [Background](#background)
+* [Summary](#summary)
+* [Deprecation Process](#deprecation-process)
+  * [`feature-exists`](#feature-exists)
+
+## Background
+
+> This section is non-normative.
+
+`feature-exists` is ill-defined as nothing specifies what each feature actually
+requires to be considered as supported. The shared `sass-spec` testsuite does
+not cover this either. Thus, no new feature identifier has been added in it
+for years.
+
+New Sass features essentially fall into one of three categories:
+
+1. New built-in functions or mixins, which are easy to detect using other
+   `sass:meta` functions.
+2. Language-level features which are relatively easy to detect on their own
+   (for example, first-class calc can be detected with calc(1 + 1) == 2).
+3. New syntax which can't even be parsed in implementations that don't support
+   it, for which feature detection isn't particularly useful anyway.
+
+## Summary
+
+> This section is non-normative.
+
+The `feature-exists` function of `sass:meta` and its global alias will be
+removed without any direct replacement.
+
+## Deprecation Process
+
+The `feature-exists` function will be supported until the next major version
+with a deprecation warning.
+
+### `feature-exists`
+
+During the deprecation period, when invoking the `feature-exists` function,
+emit a deprecation warning named `feature-exists`.