Immutability makes it easier to scale writes in a relational DB, but comes at the expense of more complexity and difficulty scaling reads. It's also problematic with regulation like GDPR / "right to be forgotten".

Immutability is definitely not a requirement for scaling a database. In my direct experience, the vast majority of large tech companies use mutable relational databases, with a write workload mixing INSERTs, UPDATEs, and DELETEs.