Yep, this is definitely just speculation, but I think this is it. Code/queries that worked fine at small load for private subs just doesn't work at scale when tons of subs are private.
Isn't the source code of Reddit on GitHub? At least it used to be...
They have two older versions up but nothing recent.
Wouldn't want the recent stuff anyways (except to pontificate)
They stopped updating/supporting the code about 6 months before the big redesign (last push Oct '17, redesign launched Apr '18). Call me a bit of a conspiracy-theorist, but they just happened to raise $200 million on their Series C less than 3 months prior to abandoning their commitment to open-source.