It's more "best practice, but uncommon", in my experience at least. And that's
the problem - it most certainly is not required and I see little if any chance
of getting consensus on recommending it, given that we cannot seem to get
consensus on lots of even simpler, more obvious stuff.
Part of the problem may be that RFC 2119 equates "RECOMMENDED" to
"SHOULD", which essentially means "you MUST do this unless you have a
good reason to make an exception". So there is an unfortunate tendency
for anything that is merely recommended to be stated in such a way as to
carry the weight of a requirement.
For operational recommendations to gain consensus I think we'll need to
find some language that conveys a greater range of {en,dis}couragement
than just MUST/SHOULD/MAY/SHOULD NOT/MUST NOT. maybe
MUST
SHOULD
STRONGLY ENCOURAGED
ENCOURAGED
MAY
DISCOURAGED
STRONGLY DISCOURAGED
SHOULD NOT
MUST NOT
?
I also think we'll need to find language that conveys a sense of
timeframe. maybe IMMEDIATELY/ASAP/WHEN PRACTICABLE/BY {date} ?