I recently opened an Illustrator file made by my employer's previous graphic designer and found that it had these rulers along the side that have the appearance of guides, but didn't go away when I hid guides, smart guides, or rulers. They're not a problem and I don't need to recreate them, I'm just wondering if anyone has any idea what they are because I've never been them before.
Those are called Video Rulers
They can be turned on/off under View -> Rulers -> Video Rulers
You can also turn on/off various settings for these rulers based on the artboard options.
Go to Window -> Artboards
Click on the dropdown to select Artboard Options...
Under Display there are more options (checkboxes) for the video rulers like showing video safe areas.
The document has been set up be optimised for creating video assets using the Video And Film Document profile.
More info here: https://helpx.adobe.com/illustrator/using/files-templates.html