Apple has announced that the forthcoming macOS Mojave will discontinue support for the Back to My Mac service first introduced in Mac OS X 10.5 Leopard. A recently published knowledge base document lays out alternatives that offer much of the same functionality.
Back to My Mac made it easy to retrieve files or screen share with another Mac, no matter where it was, but other technologies have more or less obviated the nature for the service, which always was a bit on the unreliable side. Apple’s suggesting the use of iCloud Drive if you need to retrieve files remotely, and macOS’s Screen Sharing technology appears to allow for remote access without the need for further configuration.
This won’t cover every capability Back to My Mac provided, but given the feature’s flakiness, I imagine it was not in widespread use anymore.