How do I revert the macOS 27 update and restore liquid glass appearance?

Reverting to an older major version of macOS is not simple. You need to boot into Recovery OS, erase your internal drive, then install the older macOS, then restore from a backup made prior to the recently applied upgrade. Even then, the firmware on the Mac may have been updated, meaning potential conflicts and a possible need for DFU mode and a Configurator revive/restore, which would need to be done via another Mac connected to yours. If that sounds complicated and risky, then you're assessing the situation correctly. Also note that in no case can you install a version of macOS that is earlier than the version installed on your Mac when it came out of the box. 

*Regarding the backup, if you want to pick up where you left off on the old version, you really need a backup made before the upgrade. If you do not have such a backup (why not?), then there are alternatives but you must be prepared to accept some work to get back to your normal. If you're in that situation, here are some strategies (and caveats):

• Manual file copying: Copy your most important files, folders, photos, and music to an external storage drive.
• Third-party backup software: Tools for backing up a Mac, such as SuperDuper or Carbon Copy Cloner can create a separate copy of your data. Do not assume that a clone created under the new major version of macOS will be bootable on your Mac.
• Cloud synchronization: Make sure cloud storage services such as iCloud, Dropbox, OneDrive, or Google Drive have finished synchronizing. Do not assume that every app’s data is stored in the cloud, as some applications store data only locally.
• Time Machine: If you have a Time Machine backup from before you upgraded to Golden Gate, you can use it to transfer compatible data after reinstalling Tahoe. When setting up the Mac from the backup, be sure to use a backup date from before you applied the major upgrade. You cannot restore from a backup made post-upgrade, and any changes since the upgrade will be lost. 

If you just applied the update, Spotlight re-indexes your Mac after a macOS update, and this process can take some time and can temporarily slow down your Mac. Give it a day, at least.

More on that from Apple: About Spotlight indexing and search results on Apple devices - Apple Support

There have been many issues reported with Mission Control and Spaces on macOS 27 Golden Gate, especially regarding the way app switching and Full Screen apps are handled. Some have been resolved by identifying and removing incompatible 3rd party extensions, others are bona fide bugs. The minor point update, macOS 27.0.1, did not appear to address the more significant issues.

Bugs should be reported to Apple here:

Feedback - macOS - Apple