diff --git a/changelog.md b/changelog.md
index 4dca16705c..83ea765b89 100644
--- a/changelog.md
+++ b/changelog.md
@@ -3,6 +3,17 @@
 
 ## Changes affecting backward compatibility
 
+- `addr` is now available for all addressable locations,
+  `unsafeAddr` is now deprecated and an alias for `addr`.
+
+- `io`, `assertions`, `formatfloat`, and `` dollars.`$` `` for objects are about to move out of the `system` module. You may instead import `std/syncio`, `std/assertions`, `std/formatfloat` and `std/objectdollar`.
+  The `-d:nimPreviewSlimSystem` option makes these imports required.
+
+- The `gc:v2` option is removed.
+
+- The `mainmodule` and `m` options are removed.
+
+- The `threads:on` option is now the default.
 
 - Optional parameters in combination with `: body` syntax (RFC #405) are now opt-in via
   `experimental:flexibleOptionalParams`.
diff --git a/compiler/commands.nim b/compiler/commands.nim
index 993b5c7237..100453c0a2 100644
--- a/compiler/commands.nim
+++ b/compiler/commands.nim
@@ -1052,7 +1052,7 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
     processOnOffSwitchG(conf, {optEnableDeepCopy}, arg, pass, info)
   of "": # comes from "-" in for example: `nim c -r -` (gets stripped from -)
     handleStdinInput(conf)
-  of "nilseqs", "nilchecks", "mainmodule", "m", "symbol", "taintmode", "cs", "deadcodeelim": warningOptionNoop(switch)
+  of "nilseqs", "nilchecks", "symbol", "taintmode", "cs", "deadcodeelim": warningOptionNoop(switch)
   of "nimmainprefix": conf.nimMainPrefix = arg
   else:
     if strutils.find(switch, '.') >= 0: options.setConfigVar(conf, switch, arg)