Document Berkeley DB 4.8 wallet compatibility

doc/dependencies.md

@@ -41,8 +41,10 @@ Modernization notes
 * Boost builds target Boost 1.91.0 through the deterministic depends path.
 * Protobuf was removed with obsolete BIP70 payment request support. Normal
   `agrarian:` URI payment handling remains available.
-* Berkeley DB remains 4.8.30 in depends for legacy wallet portability. Wallets
-  built against other BDB major versions may not be portable.
+* Berkeley DB intentionally remains 4.8.30 in depends for legacy `wallet.dat`
+  portability. Do not upgrade Berkeley DB in-place for normal releases; migrate
+  to a new wallet backend such as SQLite through a dedicated, tested wallet
+  migration path.
 
 Controlling dependencies
 ------------------------

doc/modernization.md

@@ -27,6 +27,10 @@ Obsolete BIP70 payment request support was removed to eliminate the protobuf
 and `protoc` dependency from the wallet build. Standard `agrarian:` URI payment
 links are still supported.
 
+Berkeley DB 4.8.30 is intentionally retained for legacy `wallet.dat`
+compatibility. Future wallet-storage modernization should add a dedicated SQLite
+backend and migration flow instead of changing the BDB version in-place.
+
 The currently verified binaries are:
 
 - Ubuntu daemon and command-line tools