> keep track of that metadata themselves you obviously don’t need to

likely it'd a be perf. hit in most cases. They'd have to copy to the tail end (likely) of the allocated area. Or the start and offset the pointer, they'd need to know the size of the metadata and account for that, including aligning it.Hence, the tail feels 'nicer'

It's possible to manually use mmap and forgo malloc entirely, rolling your own arena manager.