The timestamps instead of boolean thing is something good engineers stumble upon pretty reliably. One gotcha is the database might be weird about indexing nulls. I'm not going to give an example because you should really read the docs for your specific database if this matters.

The ever growing set of boolean flags seems to be an attractor state for database schemas. Unless you take steps to avoid/prohibit it, people will reach for a single boolean flag for their project/task. Fortunately it's pretty easy to explain why it's bad with a counting argument. e.g. There are this many states with booleans, and this fraction are valid vs. this many with the enum and this fraction are valid. There is no verification, so a misunderstanding is more likely to produce an invalid state than a valid state.