I don't trust ChatGPT too much as it has been wrong many times previously,, but at least it gives this long answer with different explanations on the way too, and from a first view it seems as a reasonable answer:

In Optimizely CMS, the limitation you’re encountering is a common challenge when trying to replicate object-oriented inheritance with the `ContentManagement` API and `ContentDefinition` API. Unfortunately, these APIs do not directly support complex inheritance scenarios like those you describe. However, there are strategies to implement reusable SEO properties across multiple page types using these APIs.

Here’s how you can address this:
---
### 1. **Using a Metadata Extension with Blocks**
You can define an SEO block and associate it with all your page types using the `ContentDefinition` API. While this does not directly enable inheritance, it provides a reusable way to manage SEO properties.

#### Steps:
1. Create an SEO block with the required properties (e.g., `MetaTitle`, `MetaDescription`, etc.).
2. Add a property of type `ContentReference` or `Block` to each page type definition to reference the SEO block.

This way, you centralize your SEO logic in the block and can attach it to all relevant pages. This is still manageable via the `ContentManagement` API.

---
### 2. **Use Property Groups**
`Property Groups` are a feature in Optimizely CMS that allow you to define a group of properties that can be reused across multiple content types. Unfortunately, these are not fully supported out of the box in the public API but can be mimicked using:

- Custom property registration.
- Metadata definition via the `ContentDefinition` API.

---
### 3. **Dynamic Property Injection**
Create a helper class that dynamically injects SEO properties into content types at runtime. This allows you to simulate inheritance:

- Define an interface (e.g., `ISeoProperties`) with your SEO properties.
- Use `ContentType` attributes to inject properties dynamically.

For example:
```csharp
[ContentType(DisplayName = "SEO Page", GUID = "....", Description = "Page with SEO properties")]
public class SeoPage : PageData, ISeoProperties
{
public virtual string MetaTitle { get; set; }
public virtual string MetaDescription { get; set; }
}
```

Then dynamically associate these properties via a helper using the `ContentManagement` API.

---
### 4. **Custom Initialization via API Extensions**
You could leverage event hooks or initialization modules to extend all page types with SEO properties dynamically. For example:

- Use the `IContentModelUsage` service to loop through all registered page types.
- Inject SEO properties programmatically at runtime.

```csharp
foreach (var contentType in contentModelUsageService.List())
{
if (contentType.BaseType == typeof(PageData))
{
// Add SEO properties to this content type
}
}
```

---

### 5. **Flatten the Hierarchy Using Mixins**
Mixins in Optimizely can allow you to decouple the SEO properties from the content type. Mixins let you define shared functionality without inheritance, and you can use the `ContentDefinition` API to associate them programmatically.

---

### Why APIs Limit Direct Inheritance
The limitation you are experiencing with the APIs stems from how Optimizely CMS is architected to work with dynamic content definitions. Content types in Optimizely are stored and managed as data, and this restricts classical inheritance mechanisms.

---

### Recommendation
The most effective and maintainable approach for your case would likely be **option 1 (SEO Block)** or **option 5 (Mixins)**. Both methods allow for reusability, are manageable through the APIs, and adhere to Optimizely's extensibility patterns.