Apple Developer Forum links broken after WWDC 2020 forum upgrade At WWDC 2020, Apple updated their Developer Forums.
This has broken links to the old developer forum, such as:   https://developer.apple.com/forums/message/335101
The forum link now says:

Error - Page Not Found
The page you’re looking for can’t be found

I linked that topic on Ask Different here. The link in that Question is now broken.

Were the old Apple Developer Forum topics moved to the new Forum? Was that content deleted?
The URLs were evidently changed. Is there a way to get a new URL if you have only the old URL?
Are the old topics archived anywhere? Or has all that content been deleted?
Knowing only the old URL, how can I find the migrated topic?

Same discussion on Ask Different Meta, here, for fixing these links SE-wide:
Apple Developer Forum links broken after WWDC 2020 forum upgrade
I logged in to the new Apple Developer Forum.  Under my profile, I still have links to my old topics. They do still exist in the new Forum.
The URLs have changed. They now use /thread/ rather than /message/ and the thread/message ID has changed:
OLD: https://developer.apple.com/forums/message/335101  
NEW: https://developer.apple.com/forums/thread/109652

and the same question posted to Ask Different: Why can I not set Safari keyboard shortcuts?
That's the details of the change. I don't know if there's a way to link the old and new IDs.