As a longtime leader of Pure Hackmons, I can humbly say no Pokemon is ever truly invincible in the metagame. It doesn't matter if it's Gen 5 Wondertomb, Gen 6 Sturdy Shedinja, Gen 7 Wonder Guard Burn Up, or Gen 8 Eternatus-Eternamax. Each of these Pokemon can be hit and defeated by other Pokemon in some common ways even if those ways are limited. The same would apply for every other OM format and every Smogon tier. HOWEVER, I can't always say this would apply for Pet Mods.
Universal was a Pet Mod where Snaquaza gave each legendary and mythical Pokemon primal reversions. Most of these Pokemon would get their own custom stat distribution and a different ability, while some would even get an additional type. Some of the abilities that were given to primal reversions were custom-made. Yveltal for example, had Death Force. Death Force was initially
supposed to leave the opposing Pokemon trapped if they land a KO on Yveltal. Think of it like being ingrained but without the recovery. Instead, when Yveltal was about to get KOed, Death Force would crash the game, reboot the game, and make Yveltal
literally invincible against everything. No attacks or status moves affected the Pokemon at all and the Pokemon wasn't even able to switch out! The best part is, when this was first discovered, it was in a game I was playing where I used Yveltal, pivoted into Rayquaza, and as Rayquaza was about to get KOed it became completely invincible. The catch there was, all of Rayquaza's moves were mysteriously switched to Yveltal's moves!
Here are some more replays of this:
https://replay.pokemonshowdown.com/dragonheaven-universalubers-2863 - Death Force can still be Skill Swapped. It does not affect Yveltal but it affects the Skill Swap user.
https://replay.pokemonshowdown.com/aqua-primordialpokemonubers-1421 - The format was once called Primordial Ubers. Ignore the "Terrakion" name. That's not actually PS user Terrakion. I used unicode to make it look like Terrakion at the time
I would've had the replay for when Rayquaza turned invincible but Snaquaza reset the server before that game finished :(.
After looking through the replays,
I suspect that it was only physical moves that would trigger the invincibility. All of those replays had it triggered by physical moves and there were others where special moves were used, but the ability did not trigger.
Links:
https://replay.pokemonshowdown.com/aqua-primordialpokemonubers-1420 - Articuno was able to take out Yveltal with Freeze Dry
https://replay.pokemonshowdown.com/aqua-primordialpokemonubers-1421 - It is worth mentioning that in this and the link right above, performing the same trick I used to make Rayquaza invincible did not work, and Rayquaza was hit by a special move on both occasions. I forgot what it was hit by when it triggered on Rayquaza.