docs/INCOMPATIBILITIES: describe the new meson option

diff --git a/docs/INCOMPATIBILITIES.md b/docs/INCOMPATIBILITIES.md
index b0f65748622e05acc5f5ca9fe7ab86a335ebbd90..c02394d412033f89c6cf233d7a959632efdd4bd9 100644 (file)
--- a/docs/INCOMPATIBILITIES.md
+++ b/docs/INCOMPATIBILITIES.md
@@ -7,7 +7,16 @@ SPDX-License-Identifier: LGPL-2.1-or-later
 
 # Compatibility with SysV
 
-Since systemd v260, support for SysV functionalities has been removed. The documentation below is preserved for historical references only.
+Since systemd v260, support for SysV functionality has been removed.
+The documentation below is preserved for historical reference only.
+
+A few interfaces are optionally kept for backward compatibility.
+When systemd is compiled with the `-Dcompat-sysv-interfaces=true` setting,
+legacy interfaces are provided,
+e.g. the `runlevelX.target` aliases,
+and lock directories in under `/var` and `/run`.
+This option may be extended to cover other deprecated interfaces in the future.
+
 
 ## Pre v260 state