In reply to Nick.
When I read your comment I felt like I’d read something about that before, and it looks like that’s actually included as a performance improvement in the changelog for 3.0.0:

* Performance – Removed the feature where old orders get access to new downloads on product edit. Looping potentially thousands of orders to do this is too much of a performance burden for stores and this can sometimes be unexpected behavior too. This does however updates *edited* downloads.

In other words, it’s not technically a bug….

The WooCommerce FAQ on downloadable products mentions this change as well: https://docs.woocommerce.com/document/digital-downloadable-product-handling/#section-8

And links to a plugin to restore the functionality to 3.0+: https://github.com/woocommerce/grant-download-permissions-for-past-woocommerce-orders