Is the suffix /$entity only relevant for collection-valued containment, or also for single-valued containment?

Also for single-valued containment. The suffix is omitted for singletons, but these are always top-level in the container. A single-valued containment navigation property contains a single entity, not a singleton.

(By contrast, a non-containment navigation property can target a singleton through a suitable navigation property binding.)

I disagree: a singleton is basically a single-valued containment navigation property of the entity container.

They behave the same in all other aspects, why should their context URL behave differently?

They behave the same in all other aspects, why should their context URL behave differently?

Because [OData-CSDL, section 13.3] says:

Singletons are top-level single-valued resources.

Perhaps change that?

That statement is correct. And single-valued containment navigation properties are nested single-valued resources.

So rather clarify the section on context URL and use the service-relative path of the single-valued resource as its context URL, consistently without the suffix /$entity.

@mikepizzo and @Hubert-Heijkers would expect same treatment as singletons, without suffix /$entity