The A is generally for asexual and the reason to leave it out is because asexual people do not face the same kind of direct social discrimination of the other groups represented in the non-plus part of the acronym. That's why you don't see A in this official Canadian government communication, but you might find it in say, an online essay critiquing of a movie, where one might say, for example, that asexual, bi, gay, lesbian, queer, etc characters historically been somewhat underrepresented in media.

IMO, there's no need to be the same acronym every time - context matters, like most language. You can pull stuff out of the plus as required, or just as you feel like. It's no big deal. LGBTQ+ is standardized for most people in most cases, such as the headline of the post we're discussing, even though the actual Canadian government communication it's reporting on uses 2SLGBTQI+. We know they're both talking about the same general group of persons, but thinking "what the heck is 2S?" is a large part of Canada adding that, because they want you to learn about it. Ditto for the I. Take those two away and we're back to LGBTQ+ which I don't think is considered obscure or unwieldy. A bit of extra formality is generally to be expected from government documentation, I think.

Which "they" are you talking about?

The I is intersex, not intersexual. It is a term for people who are born biologically with a mixture of male and female traits, through chromosomal abnormalities or otherwise. This is something like 1-2 percent of most human populations, which is relatively significant, but we rarely know about, talk about, or recognize those people, hence the inclusion.