To answer this question I will ask another.
What region would you put them in?
First option -Kanto- As not one of the original 151 I doubt people would allow the Meltan line to be classed as Kanto pokemon.
Second option -Alola- Meltan was not introduced until after Alola and is not in the Alola pokedex nor available in any of the Alola games, so it makes no sense to put it here.
Third option -Switch2019- If Meltan is in the next region's pokedex then that settles it, but if it isn't then that just complicates matters further.
Pokemon Go introduced Meltan, and Pokemon go organises Pokemon by region, which makes perfect sense. However organising by region is really just organising by gen but naming each gen after that generation's respective new region. Following that logic saying the Meltan line are gen 7 pokemon could be equivalent to saying they are Alolan which they are clearly not. So if the Meltan line are not Alolan then do we consider them an exception that are still gen 7 or do we consider them gen 8 pokemon that were revealed early, the best way to answer that is to wait and see.
Lets make up a hypothetical scenario. What if the meltan line was introduced mid-generation in a more traditional Pokemon remake? Say meltan was introduced in oras instead of pikavee then what region would we consider it? To me that would depend on whether it was available in xy via trade or etc, If it was it would likely be considered a Kalos pokemon even if to get it in Kalos one had to trade it over from Hoenn. What if oras couldn't interact with xy and it was thus impossible to have the pokemon in xy, in that case we couldn't consider it kalosian, right? but what would we consider it then? Hoennense? Alolan? but its not in the Alola dex!
It would be messy and so I'm hoping that meltan is in the Switch2019 dex and we can all call it a gen 8 Pokemon but as I said eariler the best way to answer that is to wait and see