I binned a page by mistake. Are its redirects gone?

Only while it is in the bin. Restore it and publish it again, and the redirects its old addresses had come back, pointing at the page, and its addresses leave Broken links. Restoring brings a page back as a draft, and they wait until you publish it. If you publish it under a new address, the one it had before redirects there too.

An old address that something else uses by then keeps it: a redirect you wrote yourself while the page was in the bin, or a page, post or category published there since. Once the page is deleted permanently, or the bin is emptied, there is nothing to bring back.