I don't think I'd use any rubber mulch on my property, ever. Same with the dyed mulch junk. According to the person at my local waste facility -- they separate the "dead wood"/pallet wood/lumber from the green waste - green waste gets shredded and composted and sold to customers, the "dead" lumber wood gets chipped and sold to companies that dye it for dyed mulches and also for "playground mulch".
In regard to your comment about chickens eating mulch - in my experience, chickens will attempt to eat basically everything, and if it fits in their beak they'll probably swallow some of it at least once
I mulch the ground inside my chicken area with arborist mulch, topped occasionally with grass clippings or chomped up green waste and plant trimmings, leaves, sometimes a hay bale or two, or a bale of pine shavings, etc. It all mixes together and is like a giant compost pile. I harvest some of the topsoil from the run and mix it into my soil blends, every shovel is loaded with worms despite chickens foraging it every day.
Out in the garden, most of my mulch is all arborist mulch - I've moved sooooo much mulch by hand over the years, lol. Chickens wreak havoc on my mulch in the orchard, so lately I've been trying to accumulate more plants to put around the orchard trees to help contain the mulch from spreading to all the mowed grass. Figure much of those plants should be things I can feed to the chickens and also for accumulating more biomass to compost, etc. Been propagating more perennial plants like comfrey and tree collards and horseradish. Also been planting extra root crops for the greens, mustard and chards, fennel, summer squash, etc. and will be putting some cucumbers (and maybe a chayote squash) on the chicken run fencing again this year for them. I also plant a cover crop mix all over the place that has different cereal grains, legumes, broad leaf, etc. and the chickens seem to like those areas of the yard - I may start doing those in nursery trays and toss them into the run every week like a fodder system.
I have planted chicken gardens that are just for the chickens. Garden plants that produce lots of biomass... rhubarb ( I can't never remember which parts of which varieties are ok)
I'm not a rhubarb person, but I heard that the foliage is high in oxalic acid content and toxic to many animal types, including chickens. Didn't know there may be some that are ok, so anyone looking at giving this to chickens may want to do homework first!