> But I simply never see any discussion or usage of the low-level queue features offered by the language, like Deques

I respectfully disagree:

1. Deques is not a "feature[s] offered by the language". Its part of an external extension (meaning it's a C compiled plug-in to the main engine). It's development is outside of the language scope, see https://pecl.php.net/package/ds

2. Deques is barely a "low-level queue feature". It's just a data structure that can be performant for some implementations of queues. Native arrays provide slower but similar features.

3. It's sane that this (Deques) implementation detail (the in-memory data structure) is less discussed than more central and generic subjects, like scaling MariaDB.

If I consider the first two problems mentioned in the article (systems for issues and for kanban), I think Deques is unsuitable to them, because it's meant to replace arrays for in-memory works. The problems require a persistent storage, and the optimization of a PHP storage for the queue is probably irrelevant.