The required laundry circuit is permitted to serve multiple receptacle outlets in the laundry. Would a gas tankless water heater mounted in the laundry area and plugged into the laundry circuit for ignition be considered a violation?
(2) Laundry Branch Circuits. In addition to the number of branch circuits required by other parts of this section, at least one additional 20-ampere branch circuit shall be provided to supply the laundry receptacle outlet(s) required by 210.52(F). This circuit shall have no other outlets.
It does not specify what, in that laundry area, can or can't be plugged into one of those receptacle outlets.
The intent is that you have enough available power to accommodate the normal loads you have in the laundry, a washing machine and an iron, mostly. I do not see small cord and plug auxiliary loads from gas fired appliances being a problem. If they plugged in a 110v electric water heater we would have to talk. That does infringe on the 50% rule for fixed in place appliances in 210.23(A)(2). Bear in mind 210.11(C)(2) does say "receptacle outlets" and "no other outlets", so you can't have any hard wired equipment on that circuit.
It's rare for a 'laundry room' to be just a laundry room. Most often, the same room will contain something else: a toilet, a furnace, the water heater, is an entryway, or is in the corner of a large open area (such as a basement or garage).
It's a situation where it's easy to create 'de minimus' violations. When there's a gas furnace, what's the harm in the igniter sharing the laundry circuit? Even though you're violating TWO 'specified circuit' code rules. In an open area, where does the 'laundry' area end? It's so easy to create a violation simply by running an extension cord to a workbench. How's the inspector going to prevent that?
I'll revert to my 'default' setting: IMO, the code has trespassed into a design issue, where it ought not be.
That "laundry area" is not really well defined tho. If the builder put in a nice counter for folding clothes (and a receptacle for the iron), the owner might decide he needs a workbench more and the missus can fold the clothes in the bedroom.