I think Column Rules in the Text Box dialog would be a winner. Easily add a vertical rule to left, right and mid gutter.
We added Column Rules functionality in InDesign 2020. Please try it out.
Adobe InDesign team
The New Column Rule Functionality gives limited choice of line designs, how can we add our own i.e. custom designed column rule lines in InDesign
Lindsey Thomas Martin commented
The behaviour when there are non-spanning footnotes in the columns is odd (and not useful): to show the rule between footnotes, one has to apply a negative value to the end of the rule and adjust this each time a footnote is added to the second column. When the value is zero, the rule is cut off above the first footnote in the second column.
Yes! I agree :)