Saw a home recently on a slab with the water heater about dead center... the heater was on a pan that drained to a gravel area about 12" square.... looks like someone just cut a hole in the slab and dug it out and then filled with rock.
Codes change all the time... sometimes things are grandfathered and acceptable.
The pan with drain for a WH has been in the plumbing code for a very long time. I would have to look back to get the exact date, but it's probably been listed for the last 25 yrs
They grandfather some here as well. Grandfather only applies to the original appliance. Once that original is replaced, the replacement has to be brought up to current code.
The grandfather clause is really up to each inspector as there is nothing about grandfather in any of the codes.
I have replaced water heaters, and called for inspection. Once the inspector arrived. He began to inspect the electrical box ,and the house wiring. The inspector made the home owner have an electrician replace the old fuse box for a breaker box. So, grandfather didn't apply here.
Once you call for an inspection, you just may open the door for all types of problems.
Most if not all states/cities,counties,etc, that have inspections have most likely adopted international codes. Adopting International codes was a way of having a standard across all states.
When I first started out, we were using the southern codes. Southern building code, southern gas /plumbing code. Then in 1997 ? came the international codes, and there all linked together from the building codes to the electrical, to plumbing,to gas to mechanical to fire. One will reference the other many times. Gas code references the fire code many times, you then have to look in the fire code to see that section. Plumbing will reference the building codes.