If you have an understanding of what leads to mandatory reporting, etc., and you think something you say is going to put you in jeopardy during a therapy session, it's absolutely a red flag. Just what in the Hell do you think you're going to say to a therapist that's going to trigger a mandatory report? Is there some huge wave of people being involuntarily committed I've somehow missed?
Therapists don't try to have people committed or make reports lightly. They're not going to try to take your guns away or have you committed because you use bad words or seem angry during a session. They're not going to report you or whatever for disclosing drug use, or an unusual (but consensual) fetish.
They won't even report you or try to have you committed if you talk about suicide unless you meet certain criteria around likely self-harm. Saying, "man, I just don't want to live anymore" isn't going to do it. Saying "I don't want to live anymore and I have a plan to end my life in the next 24 hours" might. Even then, they're going to try really hard to guide you to voluntary actions and not involuntary ones.
The mandatory reporting laws are in place for specific scenarios like imminent danger, self-harm, or child abuse.
So - if you've actually looked into this at all and think "hey, something I say in therapy is going to trigger mandatory reporting" - yeah, that's a huge, enormous, red flag.
As I said, I'm married to a therapist. I know pretty well what triggers these things, what the process is, etc. It is an enormous headache for a therapist to have to do any of this - and it's certainly not like they're getting paid extra for every patient they report. And it's not like they have a red button that sends the men in white coats running into the room.
In normal therapy it's a multi-step process of notifications and follow-ups where a patient would have ample opportunity to raise objections, etc. If law enforcement is involved, they're not going to just cuff somebody and ask questions later unless you confess to murder in a session or something. And, maybe it's just me, but I kind of think mandatory reporting laws that kick in if someone admits to murder or imminent plans to murder someone are a good thing, not a bad one.
If you're working with a therapist through a hospital or whatever, they'll have a risk management department that will be involved in these scenarios. They're not going to support mandatory reporting or involuntary committing unless they feel it's necessary. If anything, those institutions are probably going to be biased against these things because of costs.
Independent therapists are going to avoid this because it's a huge hassle for them, too. Plus it's usually going to risk their relationship with that patient and possibly put them at liability if the patient decides to sue or go after them with their certifying board. (There is also risk, of course, if they clearly have a mandatory reporting situation and don't take action.)