Going smaller than /64 often has hardware consequences beyond just assumptions from old RFCs. E.g. in most all of Broadcom's line of ASICs you can do /127s but

Going smaller than /64 often has hardware consequences beyond just assumptions from old RFCs. E.g. in most all of Broadcom's line of ASICs you can do /127s but doing so requires initializing the ASIC on boot in a way that it has a decent chunk less table space because the IPv6 entries all have to be 128 bits now instead of just 64 bits. Many vendors expose this as a config item but default it to off kind of like the non-standard "allow routes more specific than a local subnet" option. On the software side of lower end gear it can depend on how exactly they implemented the dataplane. NAT66 (or just NPTv6 in this scenario) is still discouraged today even though sometimes it's easier to just throw hands in the air for dual-wan setups like that. Some still have hopes of a rosy multipath client future but honestly if you know you aren't going to be able to use your own PA space or connecting your SD-WAN to a CDN/Cloud to do the same isn't viable for the use case then it's definitely an "eh, yeah not a great setup but not the end of the world" kind of solution just as bad as the remaining options at this point.