Google Merchant Center Next is introducing Attribute Rules, equivalent to Feed Rules in the Classic GMC. This new feature, along with the recently announced "custom data source matching" and "Supplemental data sources", aims to provide capabilities similar to the existing Feed Rules.
Although not yet available in all GMC Next accounts, the new help pages suggest a broader release in the coming months. Implementing Supplemental Feeds and Feed Rules is crucial for the broader adoption of Merchant Center Next, particularly for advanced feed experts.
The Classic Merchant Center is expected to be available for the remainder of the year but may not last much longer. For more details, refer to the official help page: Set up your attribute rules.