We found a work-around to make the B2B apps show up again in the legacy VPP store and in Apple Business Manager: mark the app as "Available on the App Store and for volume purchasing on Apple Business Manager and Apple School Manager". Then things got worse: a new version of an app was rejected by Apple review, because it was in violation of the guidelines for public apps. This was our complete reply to the rejection:
On Nov 28th we filed case number 100962656202 at Apple Developer Support - App Set Up and Distribution. This was the issue:
"Our custom B2B (name of app removed; ToolsfR) app is not visible in the iTunes Business Store, neither the old one (vpp.itunes.apple.com) nor the new one (business.apple.com). In the old one the message is 'That item is not available. The item you were searching for is not part of the Volume Purchase Programme' (which is wrong information, as the app is part of the VPP programme and assigned to the customer).
On business.apple.com the message is 'iTunes Store is currently unavailable and unable to complete your request. Try again later.'
Nothing in https://support.apple.com/en-us/HT201400 helped. We got the message from one of our customers and could reproduce it in both stores (on Windows PC's and on Macs, with all kinds of browsers).
Please resolve."
This issue has not been resolved yet. The only way we could find to make a B2B app to show up in the legacy VPP store was to mark it as "Available on the App Store and for volume purchasing on Apple Business Manager and Apple School Manager". Please contact emea_appsen_support@apple.com and make sure they resolve case number 100962656202. Once that case has been resolved change the app back to "Available for private distribution to specific organizations on Apple Business Manager". We cannot do that ourselves in Appstore Connect.
With respect to your questions: The app is not intended for the general public. The app has always been B2B. It should remain B2B, but our business customers should also be able to buy it. Just like they used to be doing before changes on Apple's side made that impossible.
See https://discussions.apple.com/thread/250911721 as well.
