Yes, very rare in any shop of factory I've been permitted to turn a board off to connect a new circuit. Personally I feel if due care and diligence is taken it can be done safely
Respect due,but that is one hell of a lot of new circuits connected,with the reason for not isolating,being more important than your safety,on every occasion.
There are industries and situations,some of which i have worked in,where shutting down sections are "forbidden",yet when they go off as a result of any other influence,the world does not end.
In the majority of production environments,it is usually the cost of lost production,or re-setting lines,which over-rides isolation.
It may be the cost of out of hours wages,for sparks working,when isolating not during production hours.
The point i am making,is these above reasons are monetary,and if you agree that working live,carries increased risk,above working dead,then this risk is taken to benefit someone else,financially.
I have worked HV,where everybody accepts these risks beforehand,and the remuneration reflects this.
To undertake work LV,live,as described by the OP,as part and parcel of a normal routine,requires the kind of consideration,i hope my post,prompts.