diff --git a/changelog/dmd.in-externd.dd b/changelog/dmd.in-externd.dd
new file mode 100644
index 0000000000..b0d2ef8b17
--- /dev/null
+++ b/changelog/dmd.in-externd.dd
@@ -0,0 +1,7 @@
+Using `in` parameters with non `extern(D)`/`extern(C++)` functions is deprecated
+
+In preparation for enabling `-preview=in` by default,
+using `in` parameters on function that have neither D nor C++ linkage is deprecated.
+Users can replace instances of `in` with either `const` or `scope const`.
+Refer to [v2.101.0 changelog's](https://dlang.org/changelog/2.101.0.html#dmd.previewInLink)
+for a full rationale.
diff --git a/compiler/src/dmd/typesem.d b/compiler/src/dmd/typesem.d
index aed7ccbd5a..71f80f7f87 100644
--- a/compiler/src/dmd/typesem.d
+++ b/compiler/src/dmd/typesem.d
@@ -1185,19 +1185,31 @@ extern(C++) Type typeSemantic(Type type, const ref Loc loc, Scope* sc)
 
                 // -preview=in: Always add `ref` when used with `extern(C++)` functions
                 // Done here to allow passing opaque types with `in`
-                if (global.params.previewIn && (fparam.storageClass & (STC.in_ | STC.ref_)) == STC.in_)
+                if ((fparam.storageClass & (STC.in_ | STC.ref_)) == STC.in_)
                 {
                     switch (tf.linkage)
                     {
                     case LINK.cpp:
-                        fparam.storageClass |= STC.ref_;
+                        if (global.params.previewIn)
+                            fparam.storageClass |= STC.ref_;
                         break;
                     case LINK.default_, LINK.d:
                         break;
                     default:
-                        .error(loc, "cannot use `in` parameters with `extern(%s)` functions",
-                               linkageToChars(tf.linkage));
-                        .errorSupplemental(loc, "parameter `%s` declared as `in` here", fparam.toChars());
+                        if (global.params.previewIn)
+                        {
+                            .error(loc, "cannot use `in` parameters with `extern(%s)` functions",
+                                   linkageToChars(tf.linkage));
+                            .errorSupplemental(loc, "parameter `%s` declared as `in` here", fparam.toChars());
+                        }
+                        else
+                        {
+                            // Note that this deprecation will not trigger on `in ref` / `ref in`
+                            // parameters, however the parser will trigger a deprecation on them.
+                            .deprecation(loc, "using `in` parameters with `extern(%s)` functions is deprecated",
+                                         linkageToChars(tf.linkage));
+                            .deprecationSupplemental(loc, "parameter `%s` declared as `in` here", fparam.toChars());
+                        }
                         break;
                     }
                 }
diff --git a/compiler/test/compilable/warn3882.d b/compiler/test/compilable/warn3882.d
index af7aed9059..0474315be5 100644
--- a/compiler/test/compilable/warn3882.d
+++ b/compiler/test/compilable/warn3882.d
@@ -12,7 +12,7 @@ void test3882()
 /******************************************/
 // https://issues.dlang.org/show_bug.cgi?id=12619
 
-extern (C) @system nothrow pure void* memcpy(void* s1, in void* s2, size_t n);
+extern (C) @system nothrow pure void* memcpy(void* s1, const void* s2, size_t n);
 // -> weakly pure
 
 void test12619() pure
diff --git a/compiler/test/fail_compilation/deprecations_preview_in.d b/compiler/test/fail_compilation/deprecations_preview_in.d
new file mode 100644
index 0000000000..33cc904e4b
--- /dev/null
+++ b/compiler/test/fail_compilation/deprecations_preview_in.d
@@ -0,0 +1,11 @@
+/*
+REQUIRED_ARGS: -de
+TEST_OUTPUT:
+---
+fail_compilation/deprecations_preview_in.d(1): Deprecation: using `in` parameters with `extern(C)` functions is deprecated
+fail_compilation/deprecations_preview_in.d(1):        parameter `__anonymous_param` declared as `in` here
+---
+*/
+
+#line 1
+extern(C) void fun1(in char*);
diff --git a/compiler/test/runnable/debug_info.d b/compiler/test/runnable/debug_info.d
index 902f2e569e..7853d9a3d9 100644
--- a/compiler/test/runnable/debug_info.d
+++ b/compiler/test/runnable/debug_info.d
@@ -27,8 +27,8 @@ else
 extern (C)
 {
     MachHeader* _dyld_get_image_header(uint image_index);
-    const(section)* getsectbynamefromheader(in mach_header* mhp, in char* segname, in char* sectname);
-    const(section_64)* getsectbynamefromheader_64(in mach_header_64* mhp, in char* segname, in char* sectname);
+    const(section)* getsectbynamefromheader(scope const mach_header* mhp, scope const char* segname, scope const char* sectname);
+    const(section_64)* getsectbynamefromheader_64(scope const mach_header_64* mhp, scope const char* segname, scope const char* sectname);
 }
 
 const(Section)* getSectByNameFromHeader(MachHeader* mhp, in char* segname, in char* sectname)
diff --git a/compiler/test/runnable/imports/link13415a.d b/compiler/test/runnable/imports/link13415a.d
index de3bbe2ac9..077671b01c 100644
--- a/compiler/test/runnable/imports/link13415a.d
+++ b/compiler/test/runnable/imports/link13415a.d
@@ -7,7 +7,7 @@ struct S(alias func)
     }
 }
 
-extern(C) int printf(in char*, ...);
+extern(C) int printf(const char*, ...);
 
 void f(int i = 77)
 {
diff --git a/compiler/test/runnable/objc_call.d b/compiler/test/runnable/objc_call.d
index 1463ac34ba..98199a0abb 100644
--- a/compiler/test/runnable/objc_call.d
+++ b/compiler/test/runnable/objc_call.d
@@ -12,12 +12,12 @@ extern class Class
 extern (Objective-C)
 extern class NSObject
 {
-    NSObject initWithUTF8String(in char* str) @selector("initWithUTF8String:");
+    NSObject initWithUTF8String(scope const char* str) @selector("initWithUTF8String:");
     void release() @selector("release");
 }
 
 extern (C) void NSLog(NSObject, ...);
-extern (C) Class objc_lookUpClass(in char* name);
+extern (C) Class objc_lookUpClass(scope const char* name);
 
 void main()
 {
diff --git a/compiler/test/runnable/objc_objc_msgSend.d b/compiler/test/runnable/objc_objc_msgSend.d
index 21fe95cbda..8b7e8ab5c4 100644
--- a/compiler/test/runnable/objc_objc_msgSend.d
+++ b/compiler/test/runnable/objc_objc_msgSend.d
@@ -3,7 +3,7 @@
 
 import core.attribute : selector;
 
-extern (C) Class objc_lookUpClass(in char* name);
+extern (C) Class objc_lookUpClass(scope const char* name);
 
 struct Struct
 {
diff --git a/compiler/test/runnable/objc_protocol_sections.d b/compiler/test/runnable/objc_protocol_sections.d
index b2869d204d..89790c1cfc 100644
--- a/compiler/test/runnable/objc_protocol_sections.d
+++ b/compiler/test/runnable/objc_protocol_sections.d
@@ -31,8 +31,8 @@ struct objc_method_description
     }
 }
 
-SEL sel_registerName(in char* str);
-Protocol* objc_getProtocol(in char* name);
+SEL sel_registerName(scope const char* str);
+Protocol* objc_getProtocol(scope const char* name);
 objc_method_description protocol_getMethodDescription(
     Protocol* proto, SEL aSel, bool isRequiredMethod, bool isInstanceMethod
 );