There are two endpoints available in keystone to manage ec2 credentials:

• /v3/users/<userid>/credentials/OS-EC2 - https://docs.openstack.org/api-ref/identity/v2-ext/ , https://github.com/openstack/keystone/blob/stable/train/keystone/api/users.py#L311..L411
• https://github.com/openstack/keystone/blob/40b7de87e04fbc49ab3a96b5114a9ab09c2734b7/keystone/api/users.py#L50-L63
• /v3/credentials support was introduced in Identity V3: Add credentials package (#1457) #1460

Creating ec2 credentials via /v3/credentials allows to specify more options like access ID and secret, while /v3/users/<userid>/credentials/OS-EC2 automatically generates access ID and secret.