The main thing here is that this is the /hj problem. When someone says "canon" in reference to Homestuck, there are several competing definitions they could potentially be trying to say, and the source of all our woes is that it's very difficult to know which one they mean without further elaboration.

Definition 1a: A work is canon if it is first-party.

(Hiveswap Friendsim is canon)

Definition 1b: A work is canon if it is either first-party or a licensed third-party work that interacts meaningfully with the continuity of a first-party work. <- Doctor Who fans are here

(The Dead Shufflers Intermission is canon)

Definition 2: An event is canon if it is considered in-universe to be canon, that is, to be relevant, essential, and true in the sense these terms are used by the cast.

(None of the Candy timeline is canon, except for [S] 8r8k and its immediate aftermath, which are made to meet all three conditions by Vriska's overwhelming Light aura)

Definition 3a: A work or event is canon if it can be reasonably expected that future first-party material will be compliant with it. <- Most fans of mixed media properties are here

(Catnapped is canon, The Skaianet Files is not)

Definition 3b: An event/account of events/piece of information is canon if evidence suggests or shows its existence/truth within the continuity of first-party material. <- Most, but not all, arguments about canon take place here

(Sonnetstuck's Detective Pony is canon)

Definition 4: An event is canon if it occurs on-screen in Homestuck (2009-2016). Optional qualifier: outside of the Credits, due to the story map giving it the act title of "Post Canon"

(Caliborn's Masterpiece is not canon)

That's six entirely different definitions right there, all in common use, and so it's inevitable that when people talk about something's positionality with respect to canon, and they can't agree on what they're referring to what they say "canon," things rapidly get confusing. The term "post-canon" itself evades such ambiguity because all but one of the definitions of canon are ongoing and so the agreed-upon meaning of "first-party works postdating Homestuck (2009-2016), i.e. the collected body of work surrounding The Homestuck Epilogues (2019)" is the only meaning that's coherent in the first place. (Though I do really like @victoria's use of the term "post-canon" to refer specifically to the Bridges and Off-Ramps philosophy that forms the foundation of the post-canon works, and it would be neat if that caught on outside this thread.) But saying something is "dubiously canon" is the /hj problem, because that phrase isn't parametrized for what meaning of "canon" it's dubious whether the described thing is part of.

Since we're theoretically here to hammer out usable definitions, here's my take:

I propose we stop using the term "canon" and its derivatives entirely except when referring either to in-universe canonicity (definition 2, which we shall represent in sentence case) or to the original Homestuck (2009-2016) (definition 4, which we shall represent in Title Case). In this way, we would say that something relevant, essential, and true is canon in lowercase, refer to the events of the Epilogues and onward as post-Canon with this awkward camel casing, and find alternative phrasing to refer to the four other definitions mentioned above. I don't have a strong grasp on what the best possible phrasing for those definitions are, but I feel like I should put this framework out there so as to avoid talking at cross purposes.

1a - This meaning is more or less trivially captured by the words "first-party" and "third-party." Any Homestuck works made by Hussie herself or the HICU are first-party, all others are third-party.

3a - I intuitively feel like "validity" or some derivation thereof should capture this one pretty well, in the sense of "is this information or account of events still valid." Dear sweet precious Swifer's original sign was valid as of the Credits, but no longer is as of Catnapped. In this sense, Detective Pony could be said to be "dubiously valid" because while Beyond Canon makes reference to a few elements from it, the reference is glancing enough that a hypothetical unrelated work sharing those broad parameters could still fit.

3b - This could be "presence" I guess???????????

With the various different categorizations defined, I would answer @kevin's classification dilemma like this:

Homestuck

first-party

canon

valid

present

Canon

Hiveswap

first-party

unknown canonicity (events in Hiveswap are of unclear relevance, essentiality, or truth)

dubious validity (it is currently unclear to what extent Hiveswap's worldbuilding reflects onto Homestuck)

unclear presence in Beyond Canon (cherub portals?????)

extra-Canon (outside of Canon but not part of the post-Canon project)

Friendsim

first-party

non-canon (stated in-universe to be part of an attempt at replacing the alpha timeline with non-canon bullshit)

dubious validity (it is currently unclear to what extent Hiveswap's worldbuilding reflects onto Homestuck)

weak presence in post-Canon (due to its direct sequel having presence)

extra-Canon (outside of Canon but not part of the post-Canon project)

Pesterquest

first-party

non-canon (stated in-universe to be part of an attempt at replacing the alpha timeline with non-canon bullshit)

weak validity (certain aspects of the writing are deliberately meant to reflect forward onto post-Canon)

present in Beyond Canon, specifically in A Threat, Sensed

post-Canon (part of the collected body of work surrounding The Homestuck Epilogues (2019))

Paradox Space

third-party (most of the anthology), first-party (Inaugural Death[...])

unknown canonicity (the Three Pillars of Canon were not public knowledge in 2014-2015 and thus PXS' authors, bar Hussie herself, were not writing with them in mind)

validity varies (assumed invalid by default unless referenced by later material)

weak presence in Beyond Canon (Inaugural Death[...], via stylistic and referential invocation in the Nepeta Intermission)

extra-Canon (outside of Canon but not part of the post-Canon project)