SERVER-84531 added this feature flag aimed at protecting the execution of collMod commands with the parameter timeseriesBucketsMayHaveMixedSchemaData that may be incompatible with previous versions. But it actually causes crashes if collMod is invoked under an FCV not supporting timeseriesBucketsMayHaveMixedSchemaData.

That feature flag can be actually be thrown away considering that SERVER-91195 introduced a novel protocol to replicate the flag in a backwards compatible fashion so that the collMod operation it is applied on newer sub-versions or ignored in previous sub-versions (the ones not including the change done under SERVER-91195).