Is it good practice to add remove table columns on the fly?

Can it ever be a good idea in practice? Maybe, there's always a rare, bizzare use case where it may make sense, given a few conditions.

But that's not the same question as "Is it a good practice?" ... No, it would be a horrible practice, because you were never meant to do that. If you think you need to do it, then probably you need something else instead, like PostgreSQL's JSON fields.