The only instances of IPv6 NAT I've seen are situations like "I have only a single IPv6 address available but need to run a VM/a Wi-Fi hotspot/...", and arguably that's a completely different matter than "I have full control over my network, but I simply can't request the address space I need because there's none left".
Practically, I've not encountered it myself. I was recently surprised to learn that even when tethering from my iPhone, client devices receive a public IPv6, so apparently mobile networks assign more than an /128 address to even single-line customers.
On fixed-line IPv6, I've also never encountered any less than an /64 network.
/64 is not enough if you need to run two networks (e.g., a guest WiFi in addition to the main one). And yet, here in the Philippines, an IPv6 subnet larger than /64 is an enterprise-only feature that is denied to home users.
You can subnet /64. You'll just lose SLAAC. (+ some idiocy around some devices that refuse to implement both DHCPv6 and manual setting of IPv6 addresses)
That's interesting. I've been tethering for years using Android phones, and using a 4G standalone router. I've never seen a client be allocated any IPv6 address in either case. I know the phone gets one for itself, but I don't think the router does. Looking at the phone's IP address status, it is given a non-public IPv4 by the mobile network (so CGNAT if it uses it), and what looks like a single IPv6 by the mobile network.