new-doc: add section 'removed featurs'

Section added to the developer guide section

src/doc/new-doc/developer-guide/index.txi

@@ -11,6 +11,7 @@
 @c * The interpreter::
 @c * The compiler::
 @c * Porting ECL::
+* Removed features::
 @c * Experimental features::
 @c * Current roadmap::
 @end menu
@@ -18,3 +19,4 @@
 @include developer-guide/contributing.txi
 @include developer-guide/objects.txi
 @include developer-guide/environment.txi
+@include developer-guide/removed.txi

src/doc/new-doc/developer-guide/removed.txi

@@ -0,0 +1,40 @@
+@node Removed features
+@section Removed features
+
+@c @menu
+@c * In-house DFFI:: Hand-written FFI assembly
+@c * In-house GC:: ECL's own GC
+@c * Green threads:: Lightweight processes
+@c * Compiler newcmp:: Experimental compiler architecture
+@c @end menu
+
+@c @node In-house DFFI
+@subsection In-house DFFI
+
+Commit @code{10bd3b613fd389da7640902c2b88a6e36088c920}. Native DFFI was
+replaced by a @code{libffi} long time ago, but we have maintained the
+code as a fallback. Due to small number of supported platforms and no
+real use it has been removed in 2016.
+
+@c @node In-house GC
+@subsection In-house GC
+
+Currently broken GC is present in files @code{src/c/alloc.d} and
+@code{src/c/gbc.d}. Native GC is replaced by a @code{boehm gc}.It is
+planned in an indefinite future to add the native GC back as a fallback
+for unsupported platforms.
+
+@c @node Green threads
+@subsection Green threads
+
+Commit @code{41923d5927f31f4dd702f546b9caee74e98a2080}. Green threads
+(aka light weight processes) has been replaced with native threads
+implementation. There is an ongoing effort to bring them back as an
+alternative interface.
+
+@c @node Compiler newcmp
+@subsection Compiler newcmp
+
+This was abandoned effort of changing the compiler architecture. Sources
+reside in @code{src/newcmp} directory, but they doesn't build nor were
+updated for a long time. This interface will be probably removed soon.