You can still use the character map to get subscript numbers in legend, as you can see below:
However, subscripts are supported only in some fonts. You can refer to this link to know which fonts support subscript.
Superscript are already supported in character map, so I think you already solved this issue.
To select the subscript and superscript do the following steps:
Open Character Map by clicking the Start button, clicking All Programs, clicking Accessories, clicking System Tools, and then clicking Character Map.
In the Font list, type or select the font you want to use (in this example I used Calibri font).
Click the subscript (number 2) you want to insert into the legend.
Click Select, and then click Copy.
Open your QGIS go to legend item and position the cursor where you want the
subscript (number 2) to appear, then paste.
In other words, you write "NO" directly in the legend, then copy the subscript number 2 from character map, and paste it directly into the legend after "NO", and do the same for superscript. You don't need to paste first into the MS Word, then copy again from MS Word to QGIS. You do everything directly in legend item in QGIS, as you can see below:
- Uncheck Auto Update
- Click on Legend item properties
- Write the desired words with special characters following the above procedures here:
Here is the final output which was purely created using font of legend item (HTML was not used), but you need to choose the proper font that support subscript and superscrip: