This has enabled itself automatically when I want to send more than three images at once via iMessage. But I don’t want this, I just want to send them via iMessage directly.
Instead of using the Photo button in iMessage go directly to the Photos app and select and Copy the images from there from the share sheet. It then pastes them in the old way and doesn’t do the iCloud link.
I don't understand why you would want to do this. Sending a link instead means less data sent over the wire, which is faster and more efficient. That also means fewer attachments in the thread, which are stored on the devices (and possibly iCloud if iCloud messages is turned on). The photos are also shared at full quality, whereas I believe iMessage (and definitely SMS) compresses them. If the recipient wants to add the photos to their library, it's done on the back end, which again is more efficient for them. And if nothing else, sending them as a link means the recipient can browse the photos in a gallery view, as opposed to multiple messages taking up viewing space in a thread.
Why would you want to disregard all that?