No, Andrew Hussie did not just canonize the headcanon because someone found a box of Toblerones. Before any of this happened, there was evidence that Hussie liked the idea of transfem June, to the point that, according to Aysha Farah (at 32:42 of this episode of the Perfectly Generic Podcast), "The only headcanon I've ever seen Andrew get excited about is (transfem) June Egbert." It's clear he's a fan and just took the opportunity of someone "declaring June real" to nod his approval and confirm her transfem identity as canon.
Hussie has declared some disgusting things as canon you know.
I know, but those were influnced by his fatphobia, racism, abelism, and lesbophobia. Plus, he didn't bring up the idea of her being transfem, the fandom did. A fan declared it canon and he confirmed it, thus making his decision an influence from the fandom. This is not another one of his disgusting canons, this is actual canonization of a transfem headcanon the fandom created.
How does this impact the original Homestuck and the future of Homestuck?
The original Homestuck will remain untouched, but Homestuck 2 will include June Egberts transition.
Homestuck 2 isn't canon though, so her transition is dubiously canon.
No, it isn't. Homestuck 2 is dubiously canon but her transition is canon. Her transition was declared canon by the fandom and confirmed by Hussie. If Homestuck 2 and the epilogues were never released, she would still be canonically transfem, she would just never get a canon story or design.
You can have fun with your headcanons while not erasing her transfem identity.
A lot of people project onto their kins/comfort characters without erasing their canon identities.
I understand being upset if you find out you can't project onto a character anymore, especially if you have a connection to them, but ignoring canon transfem representation is extremely disrespectful and transmisogynistic to the character and transfems in general.
I can't stop you, but please try to understand how many transfems and many others will find erasing a canon transfem identitiy very offensive and transmisogynistic.
This carrd is meant to educate and possibly change minds, but if it doesn't, there's nothing I can do.
Please remember though to be kind to those who do accept her canon identity, and please do not harass people.
Don't harass them, do what makes you comfortable (blocking, muting, etc.)
I understand being mad a people who headcanon her as transmasc, but please do not bash them or harass them. It's their headcanon, and although it is erasure, harassing them for it and stirring up drama is not good, especially if they're trans themself.
There is no alternate universe where she isn't trans. June Egbert being a transfem is a universal canon and constant. The alternate universe excuse is transphobic itself because it implies that being trans is a choice, which it isn't.
Even calling pre transition her her deadname is transphobic, they aren't separate people. The only person who can call them separate people is June herself, and we have yet to see that be canon.