The AMBackwardsCompatibilityHtmlContentTransformer class was introduced in the Adaptive Media release (~7 years ago) to manage content created before Adaptive Media was implemented. This approach helped to avoid a costly upgrade at the time. However, since much time has passed and newer content no longer requires this processing, we are disabling the AM backward compatibility HTML content transformer by default.
The primary reason for this change is performance: the content transformer can consume significant CPU resources, especially under heavy traffic.
The content transformer can be enabled in Instance Settings > Adaptive Media if necessary.