ok, I wonder if there are some drivers that don't yet do this, and thus will break. :(

Maybe put a call out for this for some wider testing?

Well, right now it's a noop (fixed in D5472)

Some review on drivers with IEEE80211_C_HOSTAP bit set (not mentioned in Test Plan):
ath(4) (seems to work without promiscuous mode (partially?):

u_int32_t
ath_calcrxfilter(struct ath_softc *sc)
{
...
	/*
	 * NB: We don't recalculate the rx filter when
	 * ic_protmode changes; otherwise we could do
	 * this only when ic_protmode != NONE.
	 */
	if (ic->ic_opmode == IEEE80211_M_HOSTAP &&
	    IEEE80211_IS_CHAN_ANYG(ic->ic_curchan))   <<< 11g only?
		rfilt |= HAL_RX_FILTER_BEACON;
...
}

wi(4) (probably, never worked (look at comment in wi_parent()), may be fixed in D5381)
ural(4), run(4), rt2560/rt2661 - looks similar to rum(4)
mwl(4), rt2860 - no idea here

In D5474#116711, @s3erios_gmail.com wrote:

Well, right now it's a noop (fixed in D5472)

and, after applying it (without this patch), I have got kernel panic (ic_promisc was decremented twice for the same vap by ieee80211_check_scan())

why not?