https://arstechnica.com/gadgets/2019/02/anti-cheat-software-causing-big-problems-for-windows-10-previews/

The Windows 10 Insider Preview Slow Ring—the beta track that's meant to receive only those builds that are free from any known serious problems—hasn't received an update for months. While the fast ring is currently testing previews of the April 2019 release, codenamed 19H1, and the even-faster skip-ahead ring is testing previews not of the October 2019 release, 19H2, but of the April 2020 release, 20H1, the Slow Ring is yet to receive a single 19H1 build.

This has prompted some concern among insiders that perhaps the ring has been forgotten about, and it has even caused a few complaints from companies that are using the Windows Insider for Business program to validate new Windows releases before their launch. Without Slow Ring builds to test, there's nothing to validate, meaning that they'll have to delay deployment of 19H1 once it ships.