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There are no content requirements or restrictions. Apple does not have access to your iCloud Photos. See the following from Apple ID & Privacy - Apple Support

iCloud protects your information by encrypting it when it’s in transit, storing it in iCloud in an encrypted format, and using secure tokens for authentication. For certain information, Apple uses end-to-end encryption. This means that, by design, only you can access this information, and only on devices where you’re signed in to iCloud. No one else, not even Apple, can access end-to-end encrypted information. For more information on iCloud Data Security, see support.apple.com/kb/HT202303.