Do you have an option to clone the existing site somewhere to test.
Allow topics to be created without a category. WARNING: If there are any uncategorized topics, you must recategorize them before turning this off.
I’m guessing this means existing posts that don’t have a category may not be seen or cause an error until they have a category assigned. If you have access to the DB, this should be easily solvable.
Guess the developers thought this would be something you would / should turn on from day 1