Apple has made some changes to the latest macOS version at the driver level. If you boot Monterey with the STEX driver loaded you will see this warning.

At some point in time, macOS will not load kernel space drivers. This is similar to 32-bit programs, macOS gave warnings that at some point in time 32-bit programs would not run, and that has come to pass.

Promise has already released a user-space driver, once loaded the Legacy System Extention warning will no longer appear. This is the official announdement.

  • PROMISE begins release of Thunderbolt™3 DriverKit device drivers for Pegasus32 Series storage systems for M1 based Mac OS (macOS 12.4 and later). Drivers built with DriverKit feature improved system security and stability. DriverKit drivers run in the user space rather than as a kernel extension, for more stable and secure operation, not to mention a smoother installation process. PROMISE Pegasus DriverKit device driver will also support macOS 13 Ventura coming in late 2022.
    PROMISE will continue to provide Kernel Extension (KEXT) driver support for Pegasus32, Pegasus3 Series and Pegasus R4i running on X86 as well as M1 based Macs. KEXT drivers have direct access to the operating system after user-approved kernel extension installation. KEXT drivers are embedded in macOS for X86 hardware; KEXT drivers for M1 (macOS 11 and later) are available for download from the PROMISE website.

User space drivers do not require selecting 'reduced security' as kernel space drivers do.

The driver can be downloaded from the Promise downloads page.