I do wonder why they decided to have have separate shell size and pin designations given there appears to be a 1:1 correlation between shell sizes and pins (i.e. the 'B' shell is always 25 pins, the 'E' shell is always 9 pins). Perhaps there was plan to have fewer pins in the same shell at some point?

it's not always 9 pins. you can get pretty creative regarding the number and type of pins (high current, coax) you can fit in that shell:

DE with 2 High current contacts:

https://de.wikipedia.org/wiki/D-Sub#/media/Datei:D-Sub_conne...

DE with 15 contacts ("VGA"):

https://de.wikipedia.org/wiki/D-Sub#/media/Datei:D-SUB_DE-9-...

VGA connectors used the same shell as the DE-9 but had three rows for a total of 15 pins.

Confusingly enough, I’ve actually seen real, properly named DB-9 connectors. They were a cheaper version of a DB-25 to DE-9 converter. Instead of combining the extra pins properly, they just had a DE-9 on one end connected to only 9 pins on the DB end. They sometimes occasionally even worked properly at low enough line rates.

DE15 and DA15 both exist and are pretty commonly used, I'm not aware of any other conflicts in normal versions.

However, you can get weird Dsub connectors with things like COAX in there, so having the shell sizes have names can be useful.

IMO, the issue is less that there is a shell designation and more that the shell designation is hard to interpret.

A single letter doesn't have a lot of meaning on its own, and the A-E order is not consistent with the E shell being smaller than all the others.

By making it fully adjacent to the 'D', it makes the letter sound like it's part of the standard's name, like the 'RJ' in 'RJ45'.

It would have been better to focus on pin count and row count, as those along with standard pin spacing drive the shell size.

D-2R-15 for a two row 15 pin connector equivalent to DA-15, D-3R-15 for a 3 row 15 pin equivalent to DE-15 / VGA.

Could trim out the 'R' and go with "D2-15" for 2 row and "D3-15" for 3 row, if brevity is preferred.

I use both DA-15 and DE-15 all the time in my line of work, and I am fastidious about using the correct terminology on all my drawings. Manufacturers are ironically some of the worst at getting this right.

I vaguely remember there being some proprietary connectors that were the same as the standard with a single pin missing.