I think you mean clearances. Clearances are the spaces between parts, tolerances are the adherence to spec for those clearances; the allowable deviation. Clearances haven't changed much for rod and main bearings since the flathead. Tolerances have improved however, as machining has improved and this means that the results are more consistent so deviation from spec is less.
Of course not all Ford engines were back-spec'd to 5w-20. There were certain exceptions, for whatever reason, which I assume had to do with the results of internal testing. The modular, being a relatively recent design, had an extremely rigid bottom-end, which means things aren't moving around and it also had a crank-mounted oil pump, which is an improvement over the cam-driven ones found on the Windsor and other engines. The rigid bottom-end, relatively low specific power density and modern oiling system made it extremely tolerant thinner lubricants, which the engine family was likely tested with quite early on.
I can't remember the last modern engine I've touched that had obviously worn rod and/or main bearings. Even my old Windsor still had fantastic oil pressure with ~200,000 miles on it and was producing significantly more power than stock at the time. All the high mileage HEMI's at work are still being fed bulk NAPA 5w-20. No idea what the oil pressure is on them, as the don't have UConnect to or the digital clusters to easily check, but I suspect it's fine.
Increasing oil pressure by bumping viscosity isn't remedying anything, it's simply masking damage/excessive wear. Maybe it will buy you a few more miles and might quiet that rod knock a bit, but if you are at the point where you have rod bearing noise, that engine is on borrowed time.
The idea is to avoid getting into that situation. Yes, there are a few engines out there that will experience excessive rod bearing wear and need this addressed (BMW, I'm looking at you) but most modern engine designs should last the useful life of the vehicle and go to the wreckers with a healthy bottom-end.