I doubt there's a full-on list since the setting name is slightly misleading. Looking at my main account from my alt, the setting seems to be a combination of "block spoilery ascensions" and "block ascensions higher than the one the other person is using".

"Block spoilery ascensions" should be self-explanatory: block any ascensions that is crucial to the plot of that Servant. For example, Abby has her 2nd, 3rd, and Final Ascensions blocked since those are huge spoilers for her story. It can even be selective as in the case of Castoria, who has her only 3rd and Final Ascensions blocked.

As for "block ascensions higher than the one the other person is using", it's also exactly as it sounds like. For example, I have Lizzy on my main set to her 1st ascension across the board. She's not exactly a spoilery Servant in any way, yet my alt has her later ascensions blocked out even though the Lizzy on my alt is in her second ascension. So because of this, technically every Servant counts as a spoiler.