You can't. I communicated with Apple Support at length. I was told that Retouch was not discontinued, and he told me he could help me restore it. I told him I had read in Support Communities it was eliminated. He told me it was not. I was put on hold multiple times while he checked with other Support representatives. He was certain nothing had been done to Retouch.
When he came back after he put me on hold the last time, he told me he had confirmed that it had been eliminated. I replied that as that was the case, I did not want Sequoia, and I asked him to walk me through how to downgrade. He told me unfortunately, once you've upgraded to Sequoia, there is no way at all to downgrade because Apple made/makes it impossible to downgrade any device that updated software to Sequoia.
He was super nice and very sincere and really wanted to help. As I've posted before, it was telling (and shocking) that multiple Apple support personnel had NO IDEA that Retouch had been eliminated. Usually they are very up to speed. He was certain (as were others he consulted) that Retouch was fine, and he (and others with Support) just needed to figure out what happened to cause it not to show up on my MacBook Pro so they could easily restore it for me.
The lack of communication across the board - including the deleting of messages here - is not good. Lack of communication with Apple Support employees, lock of communication with consumers, blocking any downgrades so you are forced to use Clean Up - or not use it on anything like you did with Retouch after you've wasted so much time having to undo its bizarrely obvious repixelation. The easy solution is restore Retouch (no cost to do that) in the next software update and keep Clean Up. Retouch was eliminated with no notice to consumers or Apple Support to force us to use Clean Up. I guess we are supposed to teach it to do what Retouch already does. (That will probably get this message taken down).
But yes, due to Apple's intentional design it is impossible to downgrade once you've updated to Sequoia. Apple Support IMMEDIATELY knew that when I asked to downgrade after they finally learned and confirmed Retouch was indeed gone. Again, they were most helpful even if they couldn't help me with Retouch. They truly wanted to be able to help me restore Retouch.