deprecate_disable: add unreachable reason

Signed-off-by: Patrick Linnane <[email protected]>

Author: p-linnane

Library/Homebrew/deprecate_disable.rb

@@ -28,6 +28,7 @@ module DeprecateDisable
     no_longer_meets_criteria: "no longer meets the criteria for acceptable casks",
     unmaintained:             "is not maintained upstream",
     fails_gatekeeper_check:   "does not pass the macOS Gatekeeper check",
+    unreachable:              "is no longer reliably reachable upstream",
     # odeprecate: remove the unsigned reason in a future release
     unsigned:                 "is unsigned or does not meet signature requirements",
   }.freeze, T::Hash[Symbol, String])