pkg/nordic_softdevice_ble: Add note on being outdated

See-Also: https://github.com/RIOT-OS/RIOT/issues/11951

pkg/nordic_softdevice_ble/README-BLE-6LoWPAN.md

@@ -7,6 +7,10 @@ In general, any device capable of running Linux operating system, can be used
 as a BLE router provided the following conditions are met:
 
 * Linux Kernel >3.18 and <=4.12 is used
+  * The nRF IoT SDK version used in this package does not yet support the
+    `BLE_6LOWPAN_LEGACY_MODE` flag, which would remove the upper version bound.
+    See [the package documentation](https://riot-os.org/api/group__pkg__nordic-softdevice-ble.html)
+    for further directions.
 * bluez, libcap-ng0, radvd tools are present.
 
 If a built-in Bluetooth device is not available then Bluetooth 4.0 compatible

pkg/nordic_softdevice_ble/doc.txt

@@ -35,4 +35,18 @@ floating point instructions.
   Hopefully, either the RIOT community adds hard-float support, or Nordic
 releases a soft-float version of the used binaries.
 
+- Outdated
+
+  This package uses version 0.9 of the nRF IoT SDK, which has been integrated
+  into the nRF5 SDK v14.1.0 in its 1.0 version, and the nRF5 SDK has since been
+  released in several new versions.
+
+  A noteworthy shortcoming due to the old version is that the IPSP
+  implementation only interoperates with Linux kernels up to 4.12.
+
+  Unless the [efforts to update the softedevice
+  integration](https://github.com/RIOT-OS/RIOT/pull/9473) are rekindled, this
+  module should not be used in new projects, and @ref pkg_nimble should be used
+  instead.
+
  */