I suspect this thread is littered with old wife’s tales and flights of fancy. Disconnect negative cable first for obvious reasons. Electrical surges when a battery is reconnected are no more than when a switch is thrown. With the amount of draw on the battery when the car is at rest would seem to me that no great surge is going to happen.
Disconnecting the battery will kill all pollution readings and these will have to be reset by driving in the prescribed manner for a quick reset or longer if driven normally.
One of the obsolete ideas is that a battery will get discharged if it is sat on a concrete floor for some time, true when battery cases were made from pitch but untrue with the polypropylene batteries of today.

Some maintenance free batteries can have the electrolyte topped up, others cannot it all depends on the caps on the cells.

Make sure you have the vent hole facing the firewall plugged and the forward facing vent hole capped with the OEM vent or a vent tube down which fluid can reach the ground without getting on the car. Nasty stuff that acid.