default or selected kernelversion does not have config value CONFIG_BCACHE_ASYNC_REGISTRAION.
Result is shown for kernelversion 5.9.15

Asynchronous device registration (EXPERIMENTAL)

configname: CONFIG_BCACHE_ASYNC_REGISTRAION

Linux Kernel Configuration
└─>Device Drivers
└─>Multiple devices driver support (RAID and LVM)
└─>Asynchronous device registration (EXPERIMENTAL)
In linux kernel since version 5.8 (release Date: 2020-08-02)  
Add a sysfs file /sys/fs/bcache/register_async. Writing registering
device path into this file will returns immediately and the real
registration work is handled in kernel work queue in asynchronous
way.