Try a search for "St Stephens Square, Norwich", which works, as does "Saint Stephens Square, Norwich". I see Nominatim more as a working reference implementation on the guidelines for OSM as defined. Admittedly not a great one, per its lack of stemming for your example.

Norfolk CC are indeed the naming authority. I just doubt the quality of their database (not the first time). I'll email them about this to better understand their intent. If they've just removed the dots for brevity, or if it's a definitive form mismatching the signs.

I've seen around 30 businesses list their addresses in Companies House with 'Saint Stephens Street', so real-world use is messy. Royal Mail lists it as "St. Stephens Street", like the signage (contracted).

Norwich CC is the signing authority, where the ground truth on most of these is arguably clearer than "Rd" or "Ave" suffixes.

Having taken a look at more street names in a similar format, what seems consistent is that on the ground signs often start "St.", sometimes with the dot under the t, sometimes without space, but all contractions for 'Saint'. The council list of streets for these appears to have standardised on excluding dots and includes space. Yet the council issues the signage. So it seems that perhaps the database reduced form isn't the ideal source for naming into OSM. I reckon the signed short form best suited to short_name tag, and council form captured into official_name where differing.

Plus note Nominatim search for "Saint Benedicts Street, Norwich" has since yielded no highway results.

You're welcome to use official_name tag for that, but really the reason for not abbreviating is to avoid losing word expansion meaning and for the render agents to compress as required. There's no guarantee the sign, nor the council, are right on this. St Benedict was a saint after all and St is the contraction. It's spoken as 'Saint' too.

osm.wiki/Names#Abbreviation_.28don.27t_do_it.29

The eastern end of St Giles Street has a bit of a disconnected spur, by the way.

Former Tesco Metro was at 125

Original building postcode added by me in June 2011. No surprise to see Robert Whittaker adding them to the closed way in May 2019. I just don't believe Royal Mail would ever be all that happy about their postcodes ending up on things other than deliverable units (typically buildings).

Don't addr:postcode and addr:street belong on the building as part of deliverable address? They're already there, but are now duplicated.

I suspect the 130 may have some from the OS OpenData used as source for the original import, or the original author was European in mindset ;)

The surrounding road speed limit just hasn't been added yet. Being highway=unclassified the author may have assumed the UK default national 60 mph may have been understood, instead of explicitly adding it in at the time.

You're within rights to change the bridge to "60 mph" if you wish, as that's a closer match to the actual legal limit than 130 km/h.

The publicly maintained section of road with speed limit is to where the asphalt changes type to privately-owned developer build. Please restore maxspeed=none tag.

This maxspeed is not wrong either. 130 default interpretation is km/h, which it is for a national speed limit. A better correction would have been maxspeed="60 mph" and maxspeed:type="GB:nsl_single"

It was wrong to remove the unlimited speed here. This is a private, unadopted road with no signage and no enforceable limit. Only driving without due care/attention is prosecutable here.

Typo in earlier post of mine, meant SE one.

I suggest surface=brick_weave then, but find it surprising if you understood my reference to way/990768300

Isn't the SW one more a highway=path foot=yes surface=asphalt kind of thing, not a formal paved route?

From taginfo:
building + addr:postcode = 47.37%
amenity + addr:postcode = 2.24%
amenity + building = 0.54%

https://en.wikipedia.org/wiki/Address
It's still rather building-centric in the definition there though. Largely since the posties needs to find the building where the mail is received, while people navigating mainly rely on postcode for locating the street.

Aren't addresses as used by Royal Mail assigned to deliverable building units though? So shouldn't that information be on the building itself, perhaps postal_code on the outline if necessary?