Kata

I think just return boolean value (0 or 1) or return boolean string ('true' or 'false') is the same if you can detemine that output will be true or false.

But for the number in the range 0-255, it will be more challenge if you have to convert value 0-255 to string of number likes '123'.

Thank you for your question :D

ok, for the strings of numbers (even if, for BF katas, it will become useless as soon as you use that in several katas because it will just consist to copy/paste bits of code from one solution to another. So for one or 2, why not. But after, it becomes useless and, in my mind, just a "painfuck" addition ;) ), but for the booleans, it clearly adds nothing to the task, yes. So why to ask for strings?

There is a difference between good difficulty and fake difficulty, aka difficulty factors that are arbitrary and unrelated to the main task.

Asking people to format results is verging at being fake difficulty. Why not output 99 bottles of beer if it's true, and output first 50 fibonacci numbers if it's false? It's still "more challenge", but everybody would hate it.