The first thing you will need to do is enable Steam Console. Make sure Steam is open, and then press the Win Key + R Key. A search bar should appear, and you will want to enter the command:
steam://nav/console

With Steam Console now open, enter the following command:
download_depot 201870 201871 3788743185036498953

You should get confirmation that the files are downloading. Once you get confirmation that the download is complete, which may take a while, you are going to want to navigate to where they were downloaded.

You will find the files in your Steam library, under:
SteamLibrary\steamapps\content\app_201870\depot_201871

Now you are going to copy all of the files in this folder, and paste them into your game's library, under:
SteamLibrary\steamapps\common\Assassin's Creed Revelations

Overwrite any files, if prompted.

This section is only relevant if you own The Lost Archive DLC. You can skip this, if you do not.
First, you must complete the previous section for down-patching the main game. Once you have done that, go back to the Steam Console and enter:
download_depot 201870 201882 6947412385247558407

Wait for the confirmation that the download is complete, and then navigate to the download location:
SteamLibrary\steamapps\content\app_201870\depot_201882

Once again, you are going to want to copy all of the files in this folder, and place them into your game's library. Overwrite any files, if prompted.

The last thing we must now do is delete systemdetection.dll, which is located in your game's library. This file is known to cause many issues on modern systems, and, for me, caused textures to constantly flicker. There are no down-sides to deleting it, as far as I'm aware.
Now you should be able to boot up your game, no problem. It is perfectly safe to launch this game through Steam, whilst on-line, as well as logging into Ubisoftconnect. You will even be able to unlock achievements, still. The only thing to be aware of, is if Ubisoft does update the game, you will likely have to go through this entire process, again. Hopefully, though, that update will fix the issue this guide was made to address.

EDIT: Some folks in the comments are saying verifying files after this final step helps. Whilst I have not tested that, if you have any issues, that's probably not a bad place to start.