It is my understanding that Subflow-Enabled customizations of workflow go through a update process when the underlying workflow is updated (per Nextworld help topic Customized workflow and base workflow updates), the same as Legacy customizations did in the past.
What is different about this process for these customizations as compared to the process for legacy customizations?
In theory, there is no difference. A base workflow update will come with a release note indicating which sequences have changed and how. A legacy customization modifies and interjects sequences and logic, so the nature of the customization dictates whether changes are necessary to uptake the base update. A subflow customization intercepts a given transition to do some processing before returning to the base workflow, so it still requires analysis to determine if changes are necessary (but less frequently, ideally).
In practice, the way that each customization needs to change to adopt updates might be different, and this is situational. Release notes should indicate in detail the nature of a workflow change, what it will impact, and how to respond to changes (like “move custom logic from sequence x to sequence y”). Most of the time, this will mean that the legacy customization and subflow need to change their references, which is the same change but accomplished differently.