I would assume any cat or any breed can relax in a person's arms. I am not sure if any other breeds are known for this trait, other than Ragdolls and Ragamuffins. However, I have seen Maine Coons become very "floppy".
Not all Ragdolls go limp as well. It really depends on the individual cat. It is very much a characteristic of the Ragdoll breed and many do show this trait, even as kittens, but IMO the Ragdoll personality shows more with age.
I have seen many beautiful, mixed breeds who have what I would call the "Ragdoll personality".
I have 2 Ragdolls & 2 RagaMuffins & I guess they forgot to get that thing that makes them go limp when you pick them up.
In general Ragdolls are laid back, floppy cats, but not all Ragdolls go limp when you hold them. My moggie Simon is the only one that lets me hold him like a baby.
There is a lot of hype out there about the Ragdoll breed. Don't get me wrong I love Ragdolls, but I don't like the tall tales. I worry that some people may get a Ragdoll believing that all the hype is true & then be disappointed & want to get rid of the cat because it doesn't live up to their expectations. They do shed, they do feel pain, their fur can mat & they don't all go limp when you hold them.
Like every one else has pointed out a cat is an indiviual and will have their own personality. They don't all go limp (mine do mostly, and as ftg's said I think it comes out more with age) my kittens mostly want to play and not be held especially not like a baby, my queen is more so now that she is getting older, and my retired pet is just a little poster child of ragdoll traits, she will just hang there dangleing off your arm and wouldn't move if you dropped her (she would think until she thumped off the floor that you would catch her)
cagnes, I forgot people claimed they didn't feel pain. One of the first stories I heard of them was that the originating breeder (Ann Baker) had been on a T.V. talk show stating they didn't feel pain and through the cat against the wall, then it got real weird, it was said they didn't feel pain because the were originated from aliens or some crazy thing, lol! Any way, They certainly feel pain so hopefully no one decides on getting one for that reason, lol.
I make sure to let any one interested in buying a kitten from me know that some things related to them are myths or they are just in general to have certain traits. For example if some one says they don't shed or matt and are hypo-allergenic, I let them know how it is. They do shed and matt but with a little grooming (less then other long haired breeds) they can be easier to manage. But, the no pain thing makes me laugh, I forgot that one, lol!