As is explained in this forum post, support for DSHOT1200 signaling was indeed removed from Betaflight due to its primary use case being devalued starting in version 4.1.
DShot1200 is officially removed from Betaflight 4.1:
Dshot1200 is only needed for 32khz looptime, and 32khz looptime isn't supported any more in Betaflight. The highest looptime in Betaflight (in BF4.1) is 8KHz, and Dshot600 is enough for 8khz looptime
Dshot1200 was not stable when used with bidirectional DShot which is required for RPM filter, which has a higher priority than DShot1200
This GitHub issue also points out that DSHOT1200 was poorly supported by BLHELI_S and BLHELI_32 ESC firmwares and wasn't really necessary to maintain.