Can I revert a new MacOS X installation using a Time Machine backup? I regularly use Time Machine to create backups of my current macbook.
I am trying to upgrade to MacOS X Big Sur, but I was wondering what happens if I don't like it?
Is it possible to downgrade back to my current version of MacOS X using a Time Machine backup from just before upgrading the system?
I do not use any specialty boot up situation. I do not boot up from external drives.
It's just my laptop with its SSD. I keep larger files in external drives but OS X doesn't get installed to those drives.
The simple answer is yes.
If you have a complete Time Machine Backup of your system, wiping the disk and restoring the computer will install the macOS version of your TM Backup. You can read more about the full procedure on Macworld or on this blog or here.
The general procedure of downgrading macOS with a TM backup is the following:

Make a TM Backup
Shut down your machine and boot into recovery mode (press cmd + r right after boot chime)
Connect your external TM Backup drive and select Restore From Time Machine Backup
Continue and select your latest Backup to restore your machine

Edit: As Tetsujin mentioned in the comments, it is very important to unplug your TM drive before upgrading to the new OS and not reconnect afterwards. Ideally you get a second drive to start new Backups for the macOS version and not let the new Mac version touch the old Backup hard drive!

It is actually even possible to downgrade your macOS version without a TM Backup. You can read more about where to get the installers and more over here: How can I download an older version of macOS/OS X?