Recover a world that won't open anymore because of a mod that was removed from Workshop

If you aren't aware, your worlds and their data is stored in SQLite format on your computer.
In order to access your world's data, you'll need the right tool, otherwise you aren't going to go far. My first choice for this case is DB Browser, simple to use and straight to the point.

First, go to https://sqlitebrowser.org and click on Download at the top of the page. Next find the download link for you respective OS. If you use Windows and don't know which one to take between 32 bits and 64 bits, then take 64 bits, because chances are you are most likely using 64 bits Windows.
If you chose the installer, run it, install the program and launch it. Otherwise, extract the contents of the zip file in a folder and then run the program.

With this, you're ready to go on to the next step.

This one should be completely straight forward, just go to your game, click Play, Creative, New Game, give your world a name you'll recognise (like "Clean World" or something like it) and click Play once again. You can then exit the world (not the entire game, it's not necessary).

To fix your world, you obviously need its file. So this part covers just that.
In order to navigate there, you'll have to press the Windows key, search for "Run" and open the Run program. Then, type precisely this: %APPDATA%\Axolot Games\Scrap Mechanic\ and press OK. This should open a folder with only one folder, the "User" folder. Open it and the "User_" followed by a bunch of numbers folder. Go to the "Save" folder and find your world.
If your world's name is "Awesome Fruits" then you'll have to find a file called "Awesome Fruits.db"
Do the same for the world you created just before, which should be called "Clean World.db"
Copy both of these and paste them in a separate folder, just for security measures in case you do anything wrong.
Now, proceed to the next part of the guide!

Once you've backed up your world files, go back to DB Browser and click "Open Database" in the upper left corner of the program, find the original "Clean World.db" file and open it.
Next, find the "Table: ChildShape" list and click on it, then find "Game" and open that table.

Next, select the mods and uniqueIds fields, right click and click Copy

Now, click "Open Database" again, and open your world's original file, find the "Table: ChildShape" list again, open it, find "Game" and open that table.
Select the mods and uniqueIds fields again, right click but this time, click "Paste"!
Finally, click "Write Changes" in the upper left corner of your screen, and close DB Browser.
Then proceed to the next step!

Finally, go back to your game, go in your Creative worlds, select the world you just fixed, select your mods once again and then load the world.
If you did everything right, your world should load properly once again and you'll be able to enjoy making contraptions once again.