The supposedly released stable Linux 5.17 kernel was delayed for a variety of reasons, and Linus chose Linux 5.17-rc8 as an additional release candidate.
Linus explained in the 5.17-rc8 announcement that since the new Spectre attack hits a variety of fixes, he felt more comfortable releasing "-rc8" instead of going straight to the final release.
The new Spectre issue is that BHI/Spectre-BHB is a derivative of Spectre V2 and affects Intel and Arm CPUs, AMD's Spectre V2 handling has also changed radically. Therefore, the Linux driver for Intel/Arm/AMD also needs to be fixed accordingly, and Linus has reserved a few days for the fix.
The Linux 5.17 stable release is now scheduled for March 20.