They became identical via commit 2c68c451. It's not clear to me what the idea behind that commit was. The commit message says:

Whether to show the complete fencing history or not can be inferred from
the status of the show flags. mon_op_fence_full_history was basically
redundant.

However, --fence-history=2 vs. --fence-history=3 previously affected the output, and it does not (and did not) affect the show flags. So if we now rely on the show flags, then that removes functionality from --fence-history.

It would be a pain to try to build the old versions before-and-after, trigger various fence events, and check the history displayed by crm_mon. Hopefully we can figure this out by inspection.