After some googling it seems that neither 2018 or 2019 banned Lycanroc Dusk but always went out of their way to mention banning Ash-Greninja.I'm honestly not sure myself but I've never seen a single Dusk Lycan used in any VGC stream or gameplay I watched, and I assumed you'd at least see a occasional one in meme teams or low ladder...
Here's an excerpt the 2019 rules in particular
- All Pokémon from National Pokédex No. 001 through 807 are allowed, with the following restrictions:
- Mythical Pokémon and Ash-Greninja are not permitted.
- Players may only include two of the following Pokémon in their teams: Mewtwo, Lugia, Ho-Oh, Kyogre, Groudon, Rayquaza, Dialga, Palkia, Giratina, Reshiram, Zekrom, Kyurem, Xerneas, Yveltal, Zygarde, Cosmog, Cosmoem, Solgaleo, Lunala, Necrozma.
- Pokémon must have the Alola symbol, showing they were caught in the Alola region.