We've updated the ranges used by Synthetics private locations. These changes were made in August 2023 and as of January 2023, the legacy IPs (those used prior to August 2023) will be releaased to Amazon. The legacy IPs could be used by others in the future.
Impact to you
There is no impact to your current synthetic monitors that use private locations, but you should check and clean up firewall and/or proxy settings in your network related to the legacy IPs by January 15 2024.
Legacy US horde IPs:
Legacy EU horde IPs: