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.
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.