"Currently, there isn't an API available to fetch space permission details. Could someone assist me in locating the appropriate API to retrieve member or group details assigned to each space?"
Refer to my answer on you post in the Confluence Questions forum.
https://community.atlassian.com/t5/Confluence-questions/There-is-no-any-api-to-get-space-permission-details/qaq-p/2543953
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