Normally you'd name the field `created_at`, `updated_at`, or similar which I think makes it very clear.

> Making a boolean a datetime, just in case you ever want to use the data, is not the kind of pattern that makes your code clearer in my opinion.

I don't follow at all, if your field is named as when a thing happened (`_at` suffix) then that seems very clear. Also, even if you never expose this via UI it can be a godsend for debugging "Oh, it was updated on XXXX-XX-XX, that's when we had Y bug or that's why Z service was having an issue".