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What safety tools can I use for a text-only horror campaign?

I'm planning on running a horror game of Call of Cthulhu 7e soon, over a text format.

I'm familiar with the concept of safety tools in RPGs, which include ways for a person at the table to indicate that they are uncomfortable with something happening, ways to prevent those situations from occuring in the first place, and RPG after-care (checking in after the session to make sure everyone's okay and having a cool down period to come out of the game).

As the planned genre is horror, I think it's particularly important to make sure that everyone's having fun and that no boundaries are crossed. As the GM, I sent out a list to the players of possible subjects that could come up in the campaign and asked for them to indicate if they were okay with it (a "consent form").  
However, most of these methods that I've seen described are for an in-person group playing in the same physical location, such as a red card that a player can place on the table.  

What other safety tools can I apply to a horror campaign that's taking place over a text-only format?