curl --location --request DELETE '/rest/api/3/field/association' \
--header 'Content-Type: application/json' \
--data-raw '{
    "associationContexts": [
        {
            "identifier": 10000,
            "type": "PROJECT_ID"
        },
        {
            "identifier": 10001,
            "type": "PROJECT_ID"
        }
    ],
    "fields": [
        {
            "identifier": "customfield_10000",
            "type": "FIELD_ID"
        },
        {
            "identifier": "customfield_10001",
            "type": "FIELD_ID"
        }
    ]
}'null