Adding a vertical divider is a great way to organize your content and make it even easier to read. This can be useful when you want to make full use of the space on a page and organize the content side by side in columns.
Despite the fact that Notion does not have an official block for adding vertical dividers, I will show a workaround that makes it appear as if the page has a vertical divider.
How To Make Vertical Divider in Notion
Create a new page or utilize an existing page in Notion to which you’d like to add a column.
After you've added a few text blocks to the page, add a Quote block and move your cursor to the left of it. You'll see a small dotted icon that will appear as a handle.
You can move the Quote block by holding down the left mouse button and dragging it free to create a column layout in Notion.

Next, click on the Quote block and press the Shift key, and then Enter key to increase the height of the vertical divider.

After you've adjusted the height, drag another of the text block to the end and create a third column.

Now, hover the mouse to the left of the Quote block till you see a light gray line.

Drag it to the side to change the width and position of the vertical divider. Next, adjust the width of the third column.

There you have it! You have successfully created a vertical divider in Notion using the Quote block.