The longer the prefix the higher its utility for user mapping but the greater the privacy erosion. RFC 7871 [11.1. Privacy] recommends that recursive resolvers truncate IPv4 addresses to at most 24 bits and IPv6 addresses to at most 56 bits in the source prefix to maintain client privacy.
Described in RFC 7871, Section 11.1.
https://github.com/miekg/dns/blob/v1.1.44/edns.go#L281-L317
https://github.com/AdguardTeam/dnsproxy/blob/master/proxyutil/udp_unix.go#L78-L87
https://github.com/AdguardTeam/dnsproxy/blob/master/upstream/bootstrap.go#L57-L78
https://dnsprivacy.org/public_resolvers/