Add an initial proposal for HLSL 202x deprecations

proposals/NNNN-202x-deprecations.md

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+
+# 202x Feature Deprecations
+
+* Proposal: [NNNN](NNNN-202x-deprecations.md)
+* Author(s): [Chris Bieneman](https://github.com/llvm-beanz)
+* Sponsor: TBD
+* Status: **Under Consideration**
+* Planned Version: 202x
+* Issues: [#300](https://github.com/microsoft/hlsl-specs/issues/380),
+  [#291](https://github.com/microsoft/hlsl-specs/issues/291),
+  [#259](https://github.com/microsoft/hlsl-specs/issues/259),
+  [#135](https://github.com/microsoft/hlsl-specs/issues/135),
+  [LLVM #117715](https://github.com/llvm/llvm-project/issues/117715)
+
+## Introduction
+
+HLSL 2021 supports syntaxes and features that either do nothing, or are
+potentially confusing. This proposal tracks a set of minimal breaking changes
+for HLSL to remove misleading or confusing functionality.
+
+## Motivation
+
+This proposal is part of a larger effort to reduce carried forward technical
+debt in HLSL compiler implementations.
+
+## Proposed solution
+
+### Removal of Effects Syntax
+
+DXC supports parsing much of the legacy FXC effects syntax, however it emits
+warnings and does not provide any behavior associated with the effects syntax.
+
+In HLSL 202x we should disable effects parsing support and allow those parsing
+failures to generate errors as would otherwise occur in HLSL.
+
+#### Examples of effects syntax
+
+```hlsl
+< int foo=1; >
+<
+  string Name = "texa";
+  int ArraySize = 3;
+>;
+
+sampler S : register(s1) = sampler_state {texture=tex;};
+Texture2D l_tex { state=foo; };
+
+int foobar2 {blah=foo;} = 5;
+
+texture tex1 < int foo=1; > { state=foo; };
+
+
+technique T0
+{
+  pass {}
+}
+Technique
+{
+  pass {}
+}
+
+int foobar5[] {1, 2, 3};
+int foobar6[4] {1, 2, 3, 4};
+```
+
+### Removal of `interface` Keyword
+
+DXC supports `interface` declarations, however the semantic utility of
+interfaces is limited. Instances of interfaces cannot be created or passed
+around to functions. They can only be used as a static-verification that all
+required methods of an object are implemented.
+
+Template-based polymorphic patterns available in HLSL 2021 and later enable many
+of the code patterns that `interface` declarations would have previously been
+used for and should be the supported path going forward.
+
+### Removal of `uniform` Keyword
+
+In DXC the `uniform` keyword is parsed and ignored. This may lead users to
+believing it has some impact when it does not. We should remove it.
+
+## Disallow `cbuffer` initializers
+
+DXC allows variables within a `cbuffer` to have initializer clauses. The
+initializer clauses are ignored, and DXC does not issue a diagnostic. In HLSL
+202x initializer clauses on declarations placed into an implicit or explicit
+`cbuffer` declaration are illegal and will produce an error.
+
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