Definitely something to look out for! Game Freak is usually pretty good with patches (they patched the Lumiose City save bug in X and Y just a week after the games came out) so I don't think this should be relevant for very much longer.
Glitch City Laboratories closed on 1 September 2020 (announcement). This is an archived copy of a thread from Glitch City Laboratories Forums.
You can join Glitch City Research Institute to ask questions or discuss current developments.
We've discovered a bug that affects the moves Memento and Parting Shot during online battles using Battle Spot. While we fix this bug, Memento and Parting Shot moves will be prohibited in Free Battles, Rating Battles, and Online Tournaments in Battle Spot. If any of the Pokémon in your Battle Team has either of these moves, you won't be able to start a battle.
Keep checking the Pokémon Global Link for a notification that this bug has been fixed. Thanks for enjoying the Pokémon Global Link and battling in Pokémon Sun and Pokémon Moon.
If Nintendo says they patched it, they did
The only video of the glitch I could find was a Twitter video, so I asked the uploader if I could upload it to my YouTube channel, and I did here.