Are soldered electrical connections code-compliant?
There are better ways

A wide variety of reliable splice techniques now exist. The reason for soldering "back in the day" was that manufacturing wasn't really up to making "wire nuts" and "Alumiconns" in billion quantity.  And labor was a lot cheaper.  Today, it would be nuts to use that technique, but I wouldn't condemn old work that has stood the test of  time.  

I expect your electrician will see it the same way; if you presented new solder work done this way, forget it, but old work would be grandfathered. 

Insulation is also a problem

The usual Home Depot advice is just put a booger of PVC electrical tape onto it. Actually, the ancients used layers of both "friction tape" (tar impregnated cloth tape) and rubber or plastic tape. The friction tape's job was to protect it from physical damage, notably bare corners or burrs from punching through the tape.  This is complicated enough that I don't advise it; in fact, I don't even like using split bolts or other uninsulated splices. When solder was used in mains wire, this work was done or overseen by a master electrician, and the electrical inspector relied as much on personal experience trusting that master.

The layers of insulation also make the joint uninspectable.  If an inspector saw a soldered rattail splice today, he'd have good reason to either assume it wasn't soldered, or that it was not properly insulated. 

"Modern" soldering won't do

In mains wiring a lot of engineering is done to assure we're not wasting copper. Wires are intended to heat up, and are generally run at sane thermal limits.   

Solder only has about 1/10 the conductivity of copper.  

Now look at how modern soldering is done.  Here's a surface mount power jack.  You see the tiny tabs meant for the solder connection? 



This is the mentality of modern, electronic soldering.  Put a tiny dit of solder there (even if you drown that tab in solder, there's just not that much tab there) and call it good.  That works because currents are small - that connector is rated for 1 amp, the limiting factor being the surface area of the solder. 

It's "tack soldering", as distinct from a thorough splice like you have there, or a Western Union splice. The problem is, when you say "soldering", everyone hears "tack soldering, like done in electronics". That is not nearly acceptable.  Yes, Western Union etc. splices are alright.  They are also a "lost art". 

I don't believe the Western Union splice is a lost art you should be re-creating, because there are better ways to do this today, and it would be difficult to re-create it properly without apprenticing under an experienced master.