How to downgrade to OS X Yosemite 10.10? (Have not yet upgraded) I have a 2015 iMac 5k running OS X Yosemite 10.10.x. I've always been nervous about upgrading an expensive machine. Now I am curious about upgrading, but I would like to find an option that would allow me to roll back to my current set up if I did.

I am aware of several options referenced online, and which I used to be familiar with, but I wonder which are relevant to me and which really would support rolling back to OS X Yosemite 10.10.


Time Machine: I take backups to an external USB, but I am not clear whether this keeps an OS backup, whether there is an option to restore the OS and, if so, whether that would also restore files. I suspect an OS downgrade is not possible and that Time Machine's forte is file and app backups.
Recovery Partition: I recall, from an exercise with a MacBook Air, the notion that my machine might have a Recovery Partition. Does it? If so, which OS is stored on it - e.g. The one shipped, or the current one? If I upgrade, what does the Recovery Partition then contain? How is it restored and, again, can you restore only the OS underneath files, and/or would this erase the whole disk?
Recovery Disk Assistant: I recall this and I think I did it with a MacBook Air - the idea that you can make a bootable USB disk containing your OS, and can restore it. This seems to appeal as it seems to be a way to snapshot my current OS. But is it still a thing? I recall reading it was Lion-specific (?). How is it made/restored - through?
OS X Recovery/macOS Recovery: I believe booting to this contains options "Restore from Time Machine Backup", (see above) "Reinstall macOS" (really, only the OS, and from where?), "Get help online" and "Disk Utility" (for manually erasing the drive and then picking one of the aforementioned options?)
Bootable Installer: Now I see pages about creating one of these for Mavericks or Yosemite. Looks more technical, on the command line. But again seems to employ using an external drive to make a backup, and restored via macOS Recovery after selection via Startup Manager or Startup Disk preferences (?).
Over-the-Internet OS download - I can't recall in which scenarios an OS version would be pulled from the internet rather than from either a partition or a USB source, for example.


What is right for me?

I am concerned about upgrading to Mojave or lower (?) and having no way back. It is often the case with Apple that, whilst docs make clear something is possible, there is a catch - eg. Not being able to roll back THAT for, or a feature supported/deprecated only for a particular version. Which of the above options are supported by which OS version is a bit of a black box, in my opinion. 

Ideally, I'd have an option to roll back the OS underneath the files or, failing that, to roll back to a snapshot of both.
Now I am curious about upgrading, but I would like to find an option that would allow me to roll back to my current set up if I did.

You can use Internet recovery to rollback to the factory installed version of macOS. From the Apple Support document, How to reinstall macOS from macOS Recovery:

macOS Recovery makes it easy to reinstall the Mac operating system, even if you need to erase your startup disk first. All you need is a connection to the Internet. If a wireless network is available, you can choose it from the Wi-Fi  menu in the menu bar. This menu is also available in macOS Recovery.
Shift-Option-⌘-R
Install the macOS that came with your Mac, or the closest version still available.

Do note to keep a Time Machine backup of handy. Once you have restored the original OS, you can recover data using the backup. Also, it's best to test the sanity of the backup before actually reinstalling the OS.