usability – The button changes its text and its action. Good or terrible?

In your example, you replace the label of the button with the only available action: that of the revocation (unsubscription).

When you start to be a bit confusing, you have a check mark icon next to the button label.

One approach is to separate them. Insist on the state "You participate" in the action.

Since the main action when the analysis of the list is registeryou can do it To unsubscribe more subtle button.

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Depending on the goals of your business, you may be able to create a subtle link if you want to mitigate the act of deregistration.

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This example highlights the current status & # 39; Participate & # 39; it's clear at a glance.

This also uses a separate language to differentiate more clearly the state of the action.