In my 15+ years of web development, there are very few things I can say are unequivocally a good idea. It almost always does depend. Storing timestamps instead of booleans, however, is one of those things I can go out on a limb and say it doesn’t really depend all that much. You might as well timestamp it. There are pl...
I think this timestamp-as-a-boolean is a good idea if the field is always going to be interpreted as either True or False and nothing more. If the field in question allows for a 3rd (uncertain) value, then using a timestamp would be extremely confusing.
And it all depends on the problem at hand. Any of those solutions can be acceptable as long as you have a well thought out model.