Is it a dark practice to automatically subscribe registered users to product updates (non-marketing) newsletter?
If people use the product, it's in their interest to know more about it, since they registered in the first place - I enjoy getting emails from products I use. If a product does not interest me any more, I simply unsubscribe.
If they signed up for their product I think it's fine. Just make sure to 1) give them a simple way to unsubscribe, 2) only email them if you truly believe it's of value to them.
I'd even go as far to say that customers expect to be informed of new product changes. They are paying for the product, so if it becomes even more useful they would want to know.
especially in UK/Europe you can't do this due to GDPR rules. But aside from that, I woouldn't do that. No-one wants to be added to lists they didn't sign up for.
Updates might be bad. But if you're launching something new or version 2.0 of earlier product then email them. You just need that 1 person who is in that niche & has a big reach to get more customers. People usually forgive 1 or 2 mistakes so I don't think it would be a problem if you provide them some value another way. At least I would do so. It doesn't hurt to get more eyes.