diff --git a/TODO-1.6.md b/TODO-1.6.md
index d60fce5..a745d0e 100644
--- a/TODO-1.6.md
+++ b/TODO-1.6.md
@@ -55,3 +55,9 @@ they were unwrapped per instruction but repointing them would restore real graph
       against retired patterns before letting `leaf-readme` absolve the file), or drop the
       seed-README exemption inside retired trees. Add a lint test seeding
       `archive/notes/README.md` → must flag `retired-path`.
+      *Related learning (same cleanup, 2026-07-03):* the REST API **cannot delete
+      directories** — removing a folder's last file orphans the empty folder on disk
+      (the REST listing 404s, falsely reading as gone; Obsidian still shows it — manual
+      deletion required). Documented in `references/api-reference.md` › Deleting Files
+      and in the vault's `resources/references/obsidian-local-rest-api.md`. Any future
+      "remove retired tree" guidance must include the manual folder-cleanup step.