Dynamics 365 Customer Service doesn't have a "Developer" plan, only the trial. The only developer plan I've seen is the one associated with PowerApps https://powerapps.microsoft.com/en-us/developerplan/. This will allow you to create custom apps and use dataverse, but you won't have access to the C.S. Enterprise features like Omnichannel, for example
I hope you can verify my answer if it helps you! If you have any questions, please feel free to contact me.
Regards,
Leah
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