I think it is a matter of personal choice in both instances.

By rabbit litter, I assume you mean it is some kind of shavings or straw that is used as bedding, and that it is mixed with manure and urine? In that case, my own personal preference would be to add it to the compost pile and compost it first. The reason for composting first is that certain pathogens, like bacteria and virus organisms, could exist in the litter and it would be better to kill those first, which the heat of the compost will do over time as the manure and litter decompose. Also, urine-soaked litter might be too high in nitrogen if it is fresh.

However, if you choose to put a couple of handfuls around each plant on the soil surface as a form of a feeding mulch, it likely won't kill your plants. There is no guarantee, though, that litter that is heavy in urine won't burn plant roots. It just depends on how old the litter is. If the litter has been piled up for weeks or months, the urine likely won't be an issue and the manure itself won't be an issue anyway because it doesn't burn roots if not composted like other manures can. That leaves only the possibility of dangerous pathogens, and only you can decided whether or not to take that risk. I wouldn't.

The feed sacks are one of those things I have mixed feelings about. If they are paper only and not plasticized, they are less risky. Still, the paper, any glue or threads, the ink in the printing, etc., might contain chemicals that will go into your soil as they break down. You have no way of knowing what those chemicals are or what impact they might have. Likely there's nothing risky involved, but there's no way to know that for sure. If the bags are plasticized...like plastic-coated paper to reduce water damage, for example, I wouldn't use them in or near the garden.

Both regular newspaper (but not the glossy ad inserts) and cardboard are generally recognized as safe as a mulch layer between the soil and some other mulch material to hold them in place (like, for example, grass clippings, uncontaminated hay or straw, chopped or shredded autumn leaves, etc.). Or, both newspaper and cardboard can be composted on the compost pile. (Shredding the newspaper, stirring it into the pile, and wetting the whole the whole thing down with a hose helps keep the paper in the compost pile.)

With the feed sacks at our place, I'll put them down on the ground as a mulching/weed-blocking material between the ground and the upper layer of mulch material in beds that contain only ornamental plants, or on the ground beneath a row of containers containing vegetables or fruits, but I won't use them in the vegetable garden. That's my choice. Yours may be different, and that's fine...different strokes for different folks. I just feel like I've worked really hard for well over a decade now to build good, healthy soil and we raise a lot of the food we eat in that soil so I choose not to put anything of questionable origin or safety into it. With both regular newspaper and cardboard, studies have been done showing they're safe. With feed bags, I've never heard of studies about their safety, and it would be hard to do since they use various kinds of paper and inks. You would "think" they'd be food safe since they contain animal feed, but I don't know if it is safe to make that sort of an assumption.

Dawn