The ability to "handle" OOM in userspace, exists for a particular class of software — software that is usually configured to 1. use unbounded and unpredictable

The ability to "handle" OOM in userspace, exists for a particular class of software — software that is usually configured to 1. use unbounded and unpredictable amounts of memory, where 2. requests are entirely-sandboxed in their resource usage, but within the software itself, rather than at the OS level. Basically there's only one kind of software in this class: DBMS software. DBMSes want to be able to try to process ridiculous queries if users ask them to; and then fail in a way that only affects the processing of that query, rather than the stability of the DBMS as a whole. And they also mostly can't afford the overhead of pre-calculating just how ridiculous a query will be, before attempting it; because that calculation often requires effectively 90% of the work involved in actually running the query. For every other type of software, letting the OS handle the OOM (by killing your process) — and setting up your higher-level inter-process / inter-node architecture to be resilient to that — is the sensible approach.