I must admit, I'm not the most qualified person to answer here.

If you place the burning letter into a sealed box ( a closed system) and allow the system to carry itself to its final state, with a perfect model of the physics involved, you can trace the end state back to its original state if you know the properties of the end state perfectly because particles carry with them information regarding their previous interactions.

If you place a black hole in a box and some in-falling matter, information will be lost behind the horizon.

I must admit, I'm not the most qualified person to answer here.

If you place the burning letter into a sealed box (a closed system) and allow the system to carry itself to its final state, with a perfect model of the physics involved, you can trace the end state back to its original state if you know the properties of the end state perfectly because the constituent particles of the letter carry with them information regarding their previous interactions. That information is preserved in the system, and can be decoded.

If you place a black hole in a box and some in-falling matter, information will be lost behind the horizon, permanently so once the black hole evaporates.