It's been against the regs for at least 35 years, probably longer. And whether it complied or not, it's as dangerous now as it was then.
The problem arises when some poor sod comes along to change, for example, a ceiling rose.
With the MCB off/fuse removed on the circuit being working on, and the circuit proved dead work proceeds.
If there is a load on the other circuit (the one that is sharing the neutral) full line voltage will appear on the neutral as it is pulled from is terminal. It's f***ing dangerous (to electricians, handymen, diyers and anyone else likely to work on it) because safe isolation requires two (or more) MCBs to be turned off.
Under normal circumstances it does not pose any problem to the installation user.