Your full name, employer, address, school, phone number, or financial details. Photographs of yourself or anyone else. The name of anyone real, in any context.
This is the precaution that survives everything. Privacy settings are a promise a company makes to you; not having typed something is a fact about the world. We have never regretted keeping a companion vague about who we are, and the platforms genuinely do not need any of it to work well.
It is also worth being deliberate rather than gradual here. These conversations get comfortable, and comfortable is exactly when people start filling in details they would not have entered on day one.