A plural suffix is inflectional (probably)

[zaba]

So, plural is obviously not derivational:

dog-PL 'dogs'

But what if, in a given mysterious language, it was optional? Then the word [pjec] 'dog' can be translated as singular or plural.

But can plural, then, really be said to be inflectional (given that inflection is typically not optional "by definition")

[freknu]

Optional or not, plural inflection is by definition inflectional. You can have derivations and constructs that express number, but that simply means they express plural, not that they are plural.

many dogs = "dogs" is plural
many a dog = "dog" is singular, but expresses plural
there are plenty of fish in the sea = "fish" is uncountable, but expresses plural

If that makes any sense ... :/

[Daniel]

Is it entirely optional, in every context?

The Turkish plural suffix is also optional in many contexts. I think it's still considered inflectional.

An inflectional suffix is grammatically conditioned. If you can't find evidence for that, then it's probably inflectional.

As a general rule, a plural suffix is inflectional because that's what a plural is. It's possible what you're observing isn't actually a plural suffix-- it might be a derivational suffix that indicates a group or otherwise more than one, but not in the same sense as "plural".
Edit: what freknu said.

[freknu]

It's possible what you're observing isn't actually a plural suffix-- it might be a derivational suffix that indicates a group or otherwise more than one, but not in the same sense as "plural".

Isn't that how it works in Japanese and other Asian languages? Collective derivation or whatever it's called.

Like "wareware" (duplication) or using classifiers.

[Daniel]

Sounds about right, but I don't know the exact distribution or whether it's considered to be a true "plural" or if it's inflectional.