you must upgrade to Personal Access Tokens or OAuth

We’ve released Personal Access Tokens, which – along with OAuth – replace API Keys as the method for authorizing access to your data.
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If you're using API Keys – whether to access your own Intercom data through the API or to access other people's data through an integration you’ve built – as soon as possible.
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You'll find all the information you need to upgrade here, including key dates.

If you’ve provided your API Key to a third party in order to use an integration you don’t need to take any action. Those integrations may prompt you to reauthenticate once they’ve upgraded to OAuth, which takes just a couple of clicks.