It's just a 'proper' IPv6 private address, the IPv6 equivalent of '10.'.
I say proper as a private address in ipv6 is supposed to be fd, then 40 random bits, then whatever you want. Nothing stops you from having fd::/64 as your private network though. The thinking was people who got tired of merging networks that happened to choose the same 10.x.x.x scheme and/or vpn connecting to multiple sites similarly with the same private address. So they thought it would be swell if every private network on earth would likely be unique, so they said you 'must' randomize 5 bytes of it. Then no renumbering/no conflicting vpn connections.
I do think that's broadly an issue, and unfortunate when a condition catches "cool" factor and suddenly a bunch of people want to be that. Feel like the trend started with" Asperger's" in the 90s when a certain group of people liked the concept of "Autistic, but not bad, it just means an asshole who is smart". Over time we now have ASD, ADHD, and DID as conditions that are actively desired among certain subcultures. Which really can muddy the waters when it comes to mental health diagnosis.