Hi,
I think they chose this old chip because it has much less noise than a LM317, LM78XX and many others, even much more modern chips.
For low level audio, measurement and other sensitive app like low phase noise oscillators this can be important.
I learned a lot from a german paper about regulator noise (just looking at the graphs tells you everything you need):
https://www.bartelsos.de/dk7jb.php/rauschen-von-spannungsreglernAnother aspect is a datasheet value for long time drift and tempco. The cheap 3-legged regulators can be off by 5%, have up to 10mV of noise, a bad PSRR at higher frequencies, and no long term drift specs, plus a bad tempco in comparison. I agree that a 723 is more effort, but it can make sense.
The question for me is not if a chip is old or new, but if it does the job I need. Electronic design should not be based on what is fashionable, but on specs.
There must be a reason why TI, ST and others still sell this methusalem ...
For prototyping, the availability of a DIP or TO-99 is also a plus.
Regards
Wolfgang
PS: If you want to see some 732 designs for all kings of projects:
https://electronicprojectsforfun.wordpress.com/power-supplies/and something a bit off-center:
https://electronicprojectsforfun.wordpress.com/silly-circuits/silly-circuits-a-heated-lm723-reference/