I've recently begun ghostwriting for an author in my genre. There are very clear rules on what I can't say per my contract regarding the business side of it, but I'm curious if it's a faux pas to state that I am a ghostwriter in queries, or to put that in my Twitter bio (since agents check those).
It's not a faux pas to put it in a query.
I'd be more hesitant about putting it on Twitter which is a vast and unwieldy audience.
A query is directed to one person. If you've mentioned the ghosting in your query, an agent knows you have to be careful about revealing info. S/he won't wonder why it's not in your Twitter bio.
But if it's in your Twitter bio, the vast rabble that is Twitter may ask questions you aren't able to answer.
There's a downside to putting it on Twitter that is not outweighed by anything positive.
When you put it in your query, it's a writing credential, and belongs in the last paragraph of your query.
Be VERY careful about what you put down in writing. Some ghost writing contracts have very strict non-disclosure clauses.