The large majority of FAA rules are on an honor basis. Also, even if
violated, there is a "no harm, no foul" attitude in that controllers
don't "turn in" pilots (and vice versa). All bets are off if there is
an accident or an ATC alarm goes off.

But generally, almost all pilots in the US adhere to the rule that if
there are instrument conditions then you have to fly by IFR flight
rules and that includes landings done only under Instrument procedures.
There is an exception, that in Class G airspace you can FLY without an
IFR clearance, but that does not include landings. At any rate, landing
at an airport with low ceilings and coming down through a low cloud
deck just isnt done without an ATC clearance and an instrument
approach.

It is legal to fly under a cloud deck, even as a low as 600' or so
under SOME conditions. (I once legally flew at 500' MSL under a 600'
cloud deck for 200 miles along the Alaskan west coast). Also you can
get "special VFR" clearance in Class E, D and C surface areas but you
have to stay VFR. So there are some ways to fly in marginal VFR
conditions. But none of this includes flying in the clouds on a
non-instrument approach. You can't really legally do that and it's very
dangerous.

So I would say, yes, it is enforced. I can tell you that violating it
isn't condoned amongst the pilots I have ever met. IMC means instrument
approach if the ceilings are low enough that the regs require it. No
exceptions!