Reverting to an older operating system - will I lose my current photos?

One important caveat.

Time Machine Utility will only backup the User Account(s) ie "Home Folder " and does not contain a Full and Functional Working Copy of the Operating System

The above stated in macOS 10.15 Catalina and all versions newer.

So basically if the user Intends to use Time Machine Backup to Revert to an older Operating System - it will not work

The user would need to Erase the exiting Whole Drive and reinstall the Older Operating System as New

The iCloud aspect of your question

Whatever is presently in iCloud should not be affected and should still be available if and older version of macOS is installed.

Though, Pictures stored iCloud maybe in a too New of File Format for the Older version of macOS to understand.

Would suggest Exporting ALL the picture presenting in the Photos Application is a more common files type like on JPEG or PNG Format

Place them on a reliable External Drive.

This way you can access them again and re-import them again into the Older Operating System

If you have iCloud Library enabled on your Mac you can recover all photos and videos from the iCloud Library by following this procedure:

1 - after reinstall of the system and migration of the user data, create a new, empty Photos library.

2 - make the new library the System Photo Library in Photos' General settings pane.

3 - enable iCloud Library.

4 - It will take quite a while to download all of the photos and videos to your iMac. Depending on the size of the library it could take a couple of weeks or more.

Keep Photos open but in the background when using the iMac and leave the iMac on all night with sleep turned off but the screensaver on. Remember "patience is a virtue" and in this case a must.

The user may consider reading this article from Apple

Excerpts from below link

" If you made a Time Machine backup of your Mac, Migration Assistant can use that backup to restore your personal files, including apps and everything in your user account. "

1. " If you need to reinstall macOS, do that before continuing. For example, if your Mac starts up to a flashing question mark, you need to first reinstall macOS. "

Restore your Mac from a backup - Apple Support

Time Machine stopped backing up the system volume in Big Sur, not Catalina, and therefore can't be used to downgrade or revert it.

(236874)

Yes and for some context.

macOS 10.15 Catalina specific.

Apple made a secondary Volume ( NOT Partition ) which was where the Operating System Resided. It was called Macintosh HD.

That Volume was set as Read ONLY to protect the OS from the user tinkering around in it and making Modification.

The User Volume was called Macintosh HD-Data and that specific volumes was backed up using TM Utility

Now and moving forwards to macOS 11 Big Sir and all successor versions of macOS, the Macintosh HD is Sealed and Read Only Volume that can not be opened by the User nor by Third Party Applications.

The only Entity that can open and modify or alter this Volume is Apple.

That would occur when a update or UpGrade is performed.

So based upon this Technical Facts - Time Machine Utility would not be able to Unseal the Volume to make a full working copy of Both the Operating System and the User Volume

It can only backup the User Account Folder ( Home Folder )

As a More Learned College asserts differently regarding Catalina and TM Backup ability to have Full and Functional Working copy of the Operating system in Catalina - I am in no position to contradict this