Indeed, this was changed in 6.0. But is it happening in 8.0.0-rc.2 or in 6.12?

I faced the problem in 8.0.0-rc.2. Now, I verified that this problem already started with 6.12.1.
I'm not that sure whether the changes there are really a bugfix. I've seen so many issues here with that release.

AssertionOptions.CloneDefaults() [...] AssertionConfiguration.Current.Equivalency.CloneDefaults().

Given this is more of an extensibility feature, do you think it is worthwhile to add that to migration guide? It is mentioned in the breaking changes.

I only found ... Renamed "...AssertionOptions" to "...Options". But, this does not apply.
I'm fine if the disapeared class names are either in migration guide or release notes.

One test failed because of limitation of the recursion deep. I did not find information to limited recursion deep in V8. The last change was in 6.11.0. I've no idea why this problem starts now.

What's the specific error you're getting. Because indeed, that was adjusted in 6.11.

Xunit.Sdk.XunitException
The maximum recursion depth of 10 was reached.  
The maximum recursion depth of 10 was reached.  

With configuration:
- Prefer the declared type of the members
- Compare enums by value
- Compare tuples by their properties
- Compare anonymous types by their properties
- Compare records by their members
- Include non-browsable members
- Match member by name (or throw)
- Be strict about the order of items in byte arrays
- Without automatic conversion.

Yes, the message is duplicated. No AssertionScope is used here.

Before we used AssertionScope.FailWith. Is AddPreFormattedFailure the correct replacement? I did not find any hint on this change.

No, you need to use AssertionChain as explained in the migrationguide.

Currently, the AssertionChain looks (to me) like an internal class, even it is public. I did not find conceptual documentation for it in the documentation. It only appears in the "extensibility" and the "migration" section. I think (from that): AssertionChain is the (internal) structure used in AssertionScope (and may be other components) as data container.
So, it feels unexpected that functionality has been moved from AssertionScope to AssertionChain.
Depends AssertionScope on AssertionChain or vice versa?