I Red n a skoda service manual that if you want to work injection electrical system.You must remove the negative pole battery .For example clean.MAF sensor or remove and cleaning throttle. Body.Is that true? And why?

Any intervention on electrical system is a potential short-circuit scenario.

You don't want welding with your screwdriver, do you?

As above; also it protects the sensitive electronics in the Engine Management System, any airbag or seatbelt pre-tensioner systems, Anti-Lock Brake (ABS, because the acronym comes from German) system etc from getting high currrents that can destroy them.

BUT- remove the "earthy " lead ,which is usually the negative. This removes any return path for current if you short any powered lead to the chassis. Similarly ,if removing battery, disconnect negative first ,then positive .( Rule of thumb is to disconnect earthy side ,and on older cars this can be +ve first).