Note: The information below is now outdated since Reddit is now usable via IPv6 by default on all reddit.com-domains, based on a change on November 11 2021.
I recently stumbled upon this Reddit post, saying that the Domain ipv6.reddit.com is dualstacked. And indeed, you will see an AAAA response for this domain.
I just assumed that everything on Reddit could be available via IPv6, but the name servers just do not provide AAAA records for all other Reddit domains. Therefore, I checked what Reddit domains my browser connects to while surfing Reddit and added them manually to my dnsmasq configuration (on Linux):
address=/reddit.com/2a04:4e42:1b::396
address=/redditstatic.com/2a04:4e42:1b::396
address=/redd.it/2a04:4e42:1b::396
address=/redditmedia.com/2a04:4e42:1b::396
After restarting dnsmasq I could still surf Reddit as usual, but everything was transmitted via IPv6.
If you do not use dnsmasq, you can manually configure your /etc/hosts file by adding this line:
2a04:4e42:1b::396 reddit.com www.reddit.com www.redditstatic.com b.thumbs.redditmedia.com preview.redd.it external-preview.redd.it styles.redditmedia.com a.thumbs.redditmedia.com i.redd.it gql.reddit.com v.redd.it gateway.reddit.com www.redditmedia.com
Of course, these solutions are not really nice. If the IP changes or Reddit just disables IPv6 support again, you will get some problems. The only real solution is, that Reddit finally decides to enable IPv6 for everything by providing AAAA records.