I don't work at Reddit so I can only guess, but one does not have to convert most of their infrastructure to ipv6. Traffic terminates to load balancers and tho

I don't work at Reddit so I can only guess, but one does not have to convert most of their infrastructure to ipv6. Traffic terminates to load balancers and those would need ipv6 listeners. Everything behind them can remain ipv4 for the next million years or so if one was so inclined. The only gotchas would be anything inside their datacenter that hairpins back to the load balancer for whatever reason and there are easy work-arounds for that by creating WAN or internal vips that are ipv4.