What is the ruling on writing down people's names to pray for them in their absence so as not to forget them, and writing down supplications so as not to forget them, and what is the ruling on making and selling a planner to organize acts of worship for Ramadan?

Using a notebook or paper to remember acts of worship, or the names of those to be supplicated for, and so on, is an ordinary permissible means, because the default rule for customs is permissibility unless there is evidence to the contrary. Such matters do not appear to be prohibited, especially when a person needs them due to frequent forgetfulness.