Even discounting external changes, any reasonably complex system needs maintenance because time moves on and new interactions happen.
How many SSL certificates (internal or external) need re-issuing per month? Some of that can be automated, but in an organization as large as complex as Twitter some will be bespoke and manual, and a code freeze won't stop the clock.
How many new CVEs per month apply to Twitter's services and tooling? How many race conditions or other bugs are lurking, just waiting for the right time or traffic pattern to emerge? Twitter can't freeze inbound traffic without dire consequences.
Twitter is like your car, except that it's always running.
How many SSL certificates (internal or external) need re-issuing per month? Some of that can be automated, but in an organization as large as complex as Twitter some will be bespoke and manual, and a code freeze won't stop the clock.
How many new CVEs per month apply to Twitter's services and tooling? How many race conditions or other bugs are lurking, just waiting for the right time or traffic pattern to emerge? Twitter can't freeze inbound traffic without dire consequences.
Twitter is like your car, except that it's always running.