I wonder what the real reason for account deletions is. Could it be possible they have overflowed their primary / foreign key allocation necessary to support further user growth? Depending on their internal architecture, a migration to a wider key might not be feasible within the time horizon they have. Maybe it's easier to delete accounts? If they have weird sharding and a lot of microservices, this might be their o…
If you forced me to guess though, I think the most straightforward, simplest one is that threatening account deletion is a good way to force older users to become active again, which will increase engagement on their platform and allow them to deliver more ads. In other words, the same reason that Facebook will start spamming your email if you stop logging in.
I can't imagine any manager at Twitter would be upset about the idea that a substantial portion of their dormant userbase would be effectively "forced" to log in every 4-5 months or so.