Starting in 2020 Week 19 (Mid May), inline comments on revisions may now be associated with a character range within a line (or across several lines), rather than just a set of lines.

To comment on part of a line: select the text you want to comment on, then click New Inline Comment in the context menu that appears (or press the keyboard shortcut c).

When you hover over the comment, the character range the comment is associated with will be highlighted. Here's the feature in action: