What? ULA space is the IPv6 equivalent of RFC 1918 space. If you want a prefix you know will not change, then you go generate you a /48 and use that on your internal network(s).
Why on earth would people expect ISPs that have a "We won't give you a guaranteed-static IPv4 address." policy to have a "We will give you a guaranteed-static IPv6 prefix" policy? Thinking that way is madness.
Perhaps you're confused because you're looking at guidance for client machines, when you should be looking at guidance for customer edge routers? Check out RFC 7084, and in particular section 3 and its subsections, and section 4.3 and its subsections. [0]
> ...as the actual reputable address changes all the time
Are you talking about IPv6 address randomization? (AKA "privacy" addresses?) IF you are, then know that it's
a) Controlled by the operator of the computer, rather than the operator of the network it's connected to.
b) Optional, and SHOULD be able to be turned off by the computer operator.
c) RECOMMENDED that a stable address be generated for the machine, in addition to the random ones.
Thing c) is in part to deal with "But what if I want to put a AAAA into the local DNS?" problem, and in part to give expected-to-be-long-lived connections an address that the system knows will hang around for at least as long as the interface is up.
After all, unless you want to tear down the connections associated with it, you can't remove an address from an interface until those connections have closed. More addresses allocated to a host means more multicast groups joined for that host, which eats up resources on the local networking infrastructure... so (if you're using "privacy" addresses) you really, really wanna steer ephemeral connections to addresses that are temporary, and long-lived ones to ones that are not.
With what seems to be the recommended IPv6 setup at least for client machines, the Link-local address is the only stable identifier of that machine, as the actual reputable address changes all the time. So, the machine has both a link-local and a routable IPv6 at the same time, and both are going to be used. This is still different from IPv4, where link-local addresses are, again, an obscure feature that almost no one uses.