Vertical line for the the right margin in Notepad++

As you said you do not actually want to print it, you may use the Vertical Edge Settings.

Setting > Preferences > Editing > Vertical Edge Settings > Input column position(s) seperated with space, for example 80 100

Vertical lines would then be shown at the positions specified (e.g. column 80 & column 100).

You can tick Background mode if you prefer highlighted text instead of a line.

(Tested in: Notepad++ v7.8.9)

In version 8.1.3, the above is under:

Settings > Preference > Margins/Border/Edge > Vertical Edge Settings

BTW You can also customize the color of that vertical bar here:

In case you are looking for this option in Notepad++ at ~v8.x

The setting is located near the center of the "Vertical Edge Settings" window under Settings > Preferences > Margin/Border/Edge:

You can type in one or more numbers (separated by white space) to show one or multiple vertical lines.

In the example below, I have a "warning" line at line 72, and a hard limit at 80.

In case you are looking for this option in Notepad++ at ~v7.x

The setting is located near the center of the "Vertical Edge Settings" window under Settings > Preferences > Editing:

You can type in one or more numbers (separated by white space) to show one or multiple vertical lines.

In the example below, I have a "warning" line at line 70, and a hard limit at 80. Hope this helps.

To get a vertical line, go to Settings > Preferences > Margins/Border/Edge and type the line width in Vertical Edge Settings as a number:

Background mode modifies the color of the text instead of adding a line:

You can have multiple lines by typing multiple numbers, which makes Background mode impossible though:

To change the color of the line, go to Settings > Style Configurator. In Language: Global, Style: Edge colour's Foreground colour is what controls this.

This also affects the Background mode:

(This is as of Notepad++ v8.5.4 Jun 17 2023)