Thank you very much for your responses. Here are mine responses for your questions and a few comments:

I also think this idea is dysfunctional. […] That's kind of a big change in how `final` works.
Modern C++ compilers use LARL as resolving ambiguity may (and often do) require a separate pass. Or many passes. In fact, C++ can require an infinite lookahead.
Unconditional final remains the same, conditional final may require additional pass, same as any other `constraint-logical-or-expression`.
I could call it a big change before introducing `constraint-logical-or-expression` in C++20. But once we have concepts, I don't see the need for any change.
I am surprised about feedback as it seems you wrote about resolving condition at first pass and forgot  `constraint-logical-or-expression`  can require multiple passes or forgot I've proposed condition to be `constraint-logical-or-expression`  and not `constant-expression`.

On a similar note, how is your `is_final` supposed to function at all?
1) unary is_final (is_final<class T>) should be true if the function is unconditionally final (single one word final specifier as before proposal). In other words: true if no class can derive from T.
2) relation is_final (is_final< class Base, class Derived>) should be true if Derived derives from Base and either Base is unconditionally final or it's conditionally final and `constraint-logical-or-expression` of `final-clause` is true (optionally with Derived temporary used as `identifier` in `final-head`). In other words: true if Derived can derive from Base.

It could be also split into two separate type traits like is_unconditionally_final and is_conditionally_final, but it could break backwards compatibility.

I have never *heard *of "conditional inheritance."
Just first few googled examples:
https://stackoverflow.com/questions/5040831/restrict-the-classes-that-may-implement-an-interface
https://stackoverflow.com/questions/5767160/allowing-implementing-interface-only-for-specific-classes
https://stackoverflow.com/questions/26465237/allow-a-mock-class-to-inherit-from-a-final-class
https://stackoverflow.com/questions/33007574/restrict-classes-that-can-implement-interface-in-java
https://stackoverflow.com/questions/40094218/can-i-limit-which-classes-can-implement-an-interface
https://stackoverflow.com/questions/40967838/how-to-restrict-the-interface-implementation-in-c-sharp
https://softwareengineering.stackexchange.com/questions/316206/how-to-make-interfaces-usable-for-special-classes-only
There are many requests for a conditional inheritance across many languages.
Some languages already supports limited functionality to my proposal, restricting inheritance:
- PostSharp has `InternalImplement` attribute, see "Restricting Interface Implementation": https://doc.postsharp.net/control-implementation
- Kotlin allows inheriting from sealed in same file: https://kotlinlang.org/docs/reference/sealed-classes.html
This pybind commit is a great example of why my proposal is useful: https://github.com/pybind/pybind11/pull/2151/files

At this point, the name `E` is undeclared, so this can't possibly work.
It does not need to be declared, as it's a `constraint-logical-or-expression`. It needs to be declared for final conditioning only,
Look for this example (not proposal related, but also uses `constraint-logical-or-expression`):
```
template<class T>
  requires std::is_same_v<T, C>
void f();
struct C {};
```
Here, C in the second line of code also is not defined (not ever declared) but it's ok, as it's found on the second pass.
The Same rules apply for my proposal, as it's also `constraint-logical-or-expression` and not  `constant-expression`.

Not okay; redefinition of struct `F`.
Indeed my bad in R0 as there is no partial classes support in raw ISO C++:
struct F : D {}; // OK: F derives from E
- struct G : D {}; // ill-formed: G does not derive from E
+ struct H : F, D {}; // OK: F derives from E
+ struct I : G, D {}; // ill-formed: G does not derive from E
Also, there should be 'In case (3)', not 'In case (2)' in the Example section of proposal, and note in Wording/8 should be marked as added (green).

> Furthermore: the proposal states "Disadvantages: none", "Design trade-offs: none". While the latter may end up being somewhat accurate, the first one is complete nonsense, rendering this proposal technically ill-formed.
So will it be ok if I change 'Disadvantages' from "None" to "Proposal increases complexity of language by adding optional `final-clause`."? I've read a few proposals (both PR… and N…) and I didn't find such sentences.

So the basic question "why is this worth having?" is not answered by the proposal.
The Proposal is worth having, because it does "heavily increase compatibility correctness by inheritance constraints" by providing an ability to encapsulate interfaces. [Note: See Examples section of proposal. — end note]

Best regards.