Apple has stopped signing iOS 13.6.1 after the release of iOS 13.7 back on September 1, 2020. The move means that for those who have upgraded to iOS 13.7 will no longer be able to downgrade back to 13.6.1.
Apple routinely stops the signing (the making available on their servers) previous versions of iOS when a new version is released. This usually happens two or three weeks after a release, giving the Cupertino company a window to pull it back should there be a major issue found after it is released to the public. Doing this also helps Apple keep the number of supported versions under control for their support teams.
For most end users, a previous version being removed is not a big deal and only really impacts those that “jailbreak” their devices to run custom builds of iOS on their iPhone or iPad.
iOS 13.7 brought several tweaks to the platform but more importantly, brought the Exposure Notification API and feature to the platform. This is an important step as the world deals with the COVID-19 pandemic. Apple at the time wrote,
As the next step in our work with public health authorities on Exposure Notifications, we are making it easier and faster for them to use the Exposure Notifications System without the need for them to build and maintain an app. Exposure Notifications Express provides another option for public health authorities to supplement their existing contact tracing operations with technology without compromising on the project’s core tenets of user privacy and security. Existing apps using the Exposure Notification API will be compatible with Exposure Notifications Express, and we are committed to supporting public health authorities that have deployed or are building custom appsApple
The team at MacRumors has an excellent guide to the Exposure Notification process for those interested in understanding the mechanics of it.