Just want to ask: why WooCommerce questions are offtopic here,
Because 3rd party products/plugins are out of scope, it's not specific or unique to WooCommerce. This stack is about developers working with the open source WordPress, specifically open source WordPress.
If a question requires knowledge of a 3rd party plugin/service/theme/library then it's not strictly a WordPress question anymore, and no longer fits the scope of the stack. WPSE also isn't a support route for 3rd party products. Questions about 3rd party software be that support or developer oriented are better suited to stack overflow or the dedicated communities that exist for them.
In a lot of cases it's simply impossible to answer questions about or even see 3rd party plugins and themes without spending money. It's also sometimes the case that people get fobbed off by support and sent here, and we've caught WooCommerce staff doing that too, though usually they're trying to be helpful not realising it's off-topic.
Note that sometimes questions are on-topic even though they appear to be about WooCommerce, but there's an important distinction. E.g. there are questions where a person is trying to change a WC filter but doesn't understand how filters work, that would be on topic because it's asking how filters work in WordPress, and can be answered generally without knowing anything about WooCommerce. If that user had instead asked how that specific WooCommerce filter worked it would be off-topic because you need knowledge of WooCommerce to be able to answer the question.
but absolutely OK at Stack Overflow?
Because Stack Overflow is the catch all programming stack with the largest widest scope on the network. That doesn't mean there's people who know the answer there, but that is the purpose of the main stack overflow stack.
If you have a question that doesn't fit WordPress stack exchanges scope, it's almost always fits into Stack Overflows scope. Questions on stack overflow might get migrated elsewhere that fit better though, I've seen questions migrated to ServerFault AskUbuntu or SuperUser in the past.
My opinion: it is not the job of the plattform to ban/censor certain tags because of users bahaviour and question chances to be answered. The sense of the plattform to offer the plattform for Q&A.
And we don't, the platform has a place for questions about WooCommerce: Stack Overflow.
because of which WPSE bans the WC tag
Note that there is no specific ban on WooCommerce despite the question you linked to, or even a "ban" per say.
Questions about 3rd party plugins/themes/products you downloaded bought or installed are not in this stacks scope either. Much in the same way that a question about how to use a google REST API, how to round the corners of a
<div> in CSS, or the branding guidelines for a well known WordPress agency would also be out of scope. These all crop up for WordPress developers but that doesn't mean they're WordPress questions.
Yoast SEO, contact form 7, Astra, Divi etc are all out of scope too for the same reason, they're 3rd party stuff that's better asked in a dedicated community.
Is WooCommerce Not a 1st Party Plugin Though?
WooCommerce is a 3rd party plugin owned by Automattic and Automattics policy is that they are a 3rd party, a commercial entity).
WooCommerce itself was created by WooThemes/Woo before being purchased by Automattic ( I was also present and employed by Automattic at the time of the purchase ).
Automattic's products are sometimes misinterpreted as 1st party because the CEO has the right to use the WP trademark and is also the lead of the open source project (though wether there is a conflict of interest there is beyond the scope of this ).
The WooCommerce Tag
I'm actually responsible for the tag having
-offtopic on the end.
You see anybody can create a tag, if we delete the
woocommerce tag people recreate it, and people think it must be ok to ask WooCommerce questions because there's a tag!
We also put notes in the tag description telling people it's offtopic but people don't read it.
So if we can't delete it, and we can't prevent it being created, what's the next best thing? Putting it in the tag name itself. We can set up an alias so
woocommerce maps to
woocommerce-offtopic to avoid the recreation problem.
As the first tag we tried this with we've yet to revisit keeping the
-offtopic. This is the only tag that this has been done to, mostly to see the results, and because WooCommerce questions were a major problem at the time and vastly outnumbered all other questions about 3rd party software, combined with the fact that very very few of them even got answers.
In some cases it's worked as advertised, but sometimes people ask about WooCommerce anyway on the offchance they'll get an answer, already knowing that it's offtopic. Others still use the tag!
What About A WooCommerce Stack Exchange?
If WooCommerce doesn't fit into this stacks scope, could it have it's own stack? Yes!!
You can propose new stacks on Area 51 and that's how a lot of new stacks are created, and has even been tried in the past!
The last WooCommerce stack proposal got as far as a beta stack, but it failed to gain the necessary traction without the trial period.
The problem was that while people asked questions, almost nobody wrote answers and there was no official support. As a result it didn't gain the necessary stats to graduate to the next level.
That doesn't mean it couldn't be done again, and if it did succeed policy on this stack means we would stop closing WooCommerce questions and migrate them instead to the new WC stack.
Note that migration isn't a catch all solution though as questions that are poorly written tend to get punted back and closed by the recipients at the other end. I expect that if I as a moderator sent a WooCommerce question to stack overflow that had no research and awful wording it would be pinged right back to me and considered rude.
Can Questions Sometimes Be Brought Into Scope?
Sometimes there's a generic solution to a WooCommerce question that doesn't require WooCommerce knowledge, e.g. when a feature is actually a part of WordPress core and the question asker hasn't realised.
You'll usually see this as clarifying questions in comments, edits, and attempts to unpack a question to determine if this is possible. Not all users are receptive to this though, and it's usually not possible.