By December 31, 2025, Salesforce will no longer support Workflow Rules and Process Builder. It is therefore advised to move automation to Flow Builder by then. As per Salesforce, they want to focus more on development efforts on a cutting-edge, flexible, low-code automation solution in Flow Builder. And that is the reason why products like Workflow Rules and Process Builder are being retired.

Impact

If you have any active workflow rule or process builder in your org, which is not yet migrated into Salesforce Flow, after 2025, it will still continue to work, but there will be no enhancement or any customer support/bug fixes for these two products.

Action Needs To Be Taken

It is recommened that to migrate workflow rules and process builders into Salesforce Flow before the deadline. Salesforce provides a Migrate to Flow tool which can help you to identify scenarios and then migrate them into Salesforce Flow. If you have an org with some complex process builders and workflow rules, or even if you have multiple workflow rule and process builder per object, it makes sense to identify those existing processes and migrate them into Salesforce Flow using Salesforce Flow Best Practices. 

I have written a blog post covering Salesforce Flow Best Practices. Recommending you to please go through the post and follow the best practices. Here is the post link - Salesforce Flow Best Practices.