Keep in mind, those older devices use closed source firmware too, so you have no way of knowing if your device has a backdoor giving Ledger - or whoever - access to your seed
Yes, I know that. It's always been like that and nothing has changed nor is it likely to change unless Ledger, by some miracle, decides to open-source everything. But like you said, it will only work for the current and future software upgrades. Open-sourcing something from 1, 2, or 5 years ago won't help much if users moved away from those old firmware versions.
Personally, I think that if Ledger Recover was a thing in the past in any form, they would keep quiet about it and never announce it like they did, facing all the backlash. But we will never know.
Just curious, has ledger responded to any questions on the record about anything to do with how and where the encryption keys for these shards reside? Have they given any technical details in any literature interviews etc beyond just the "shards are encrypted" redundancy?
That's probably something you have to ask them in writing and hope that support knows how to handle your inquiry.