design – Are formal product specifications still relevant?

Confluence Pages, Jira Elements, and Agile and Kanban Documents are formal specifications of the product.

What may interest you is one of the principles of the Agile Manifesto:

[…]

we have come to value:

[…]

Software working on a complete documentation

I've already explained in a different answer what this principle means. If you assume that the only form of formal product specification is a thousand-page document that no one reads, then yes, there is no more room in mainstream development for this sort of thing (but specialized branches, software essential to everyday life, to always rely on this form of specifications).

But you still have a lot of formal product specifications, and hopefully: a product without specifications would be rather fragile.