Why did 6.1 longterm branch change the CONFIG_PREEMPT setting?

from the link you posted:

- set CONFIG_PREEMPT_VOLUNTARY as new default - PREEMPT_DYNAMIC is now available
  and desired behavior can be choosen using kernel append line

as there is no issue mentioned, I guess that’s all the information that is available.

6.5 won’t be the new LTS. 6.6 could theoretically become a LTS kernel, but so could 6.7.
For that and for the remaining question: