As reported by @creslin287 on the mailing list:
This is a friendly notice that as of today Asterisk 11 has entered
security fix only mode. From this point onward additional releases of
Asterisk 11 will no longer be made unless there is a security fix
being applied to the branch. Users of Asterisk 11 are encouraged to
move to one of the newer major versions, Asterisk 13 (LTS) or Asterisk
14, as soon as possible.
For more information on Asterisk versions and their supported lifetimes,
please see the following wiki page:
Thank you for your continued support of Asterisk!