How to get rid of this vertical line in Geany

Edit > Peferences > Editor > Display and there, deactivate the long line marker (Umbruchhilfe)

The line is, btw, not random but it marks the 80 characters limit - which is e.g. still used for Linux-Kernel code.

It's a text editor, so they're showing you where the (old standard) 72-character line break would be (in case you had line-break turned on, or plan to open the text file in a legacy text editor).

More info:

Wikipedia - Characters Per Line:

At the end of the typewriter age, most designs were geared toward 72 CPL, derived from a pitch of 12 characters per inch, multiplied by 6 inches (see for example IBM Selectric). This would ensure at least 1 inch for each margin, with the U.S. government at the time having standardized on 8 1/2×11" paper. Early computer text editors used this measure as a baseline for their terminal interfaces.

Geany's docs:

Line breaking column - The editor column number to insert a newline at when Line Breaking is enabled for the current document.

It doesn't look like there's a specific option to turn the indicator off, but perhaps setting it to column 0, or to something like 5000 might keep it out of sight at least. :)

The line breaker can be turned off by navigating to: Edit>Preferences>Editor>Display>"Untick" the box named "Long Line Marker"