When did they decide to abandon ship's bows?

[Kapitan]

First of all tear drop shape was pioneered by the K3 the first november class submarine, the K3 rendered the nautilus obsolete if you go by paper v paper side of things, it out classed nautilus it dived deeper was faster and carried more weapons and had alot more space inside.

the last boats for the russian navy built with a type 21 hull form would have been the tango class the follow on from the foxtrot.
The picture in the first post is a foxtrot, by the time the last boats entered service in the 1980's they were obsolete replaced by the tango who again was of a obsolete design most tangos didnt see past 20 years service, only one has been retained for training uses.

The introduction of the kilo made the conventional hull form extinct in the russian navy and has been adopted even in 3rd world countries, they have ditched the older hull form boats such as foxtrots (india poland) in favour of the newer more effective tear drop boats.

The thing is hydrodynamically tear drop is far better it achieves greater submerged speeds, and it also reduces noise, the foxtrot was incredibly loud as it had three screws where the kilos have just one.

I only know of about 3 navies who maintain non tear drop hull submarines, egypt (romeo improved) china (romeo improved, golf II) tiawan (whiskey)
Note the world status of these countries.