Thank you for your question/suggestion. As pointed out in a previous comment, the <u> tag is not intended to be used for general underlining of text. Instead, the HTML that is output by the Classic UI and Experience Space rich text editors should be used for this purpose.

To add custom functionality to the Classic UI rich text editor, for example, a ribbon toolbar button to insert an unarticulated annotation (<u>) tag, the extension mechanism could be used.

In Experience Space, we don't currently have extensibility support. However, a mechanism for customizing Experience Space is in active development. The rich text editor extension point, which we cannot yet commit to for the next release, should make it possible to configure the editor to output a <u> tag instead of the current markup, or to add a dedicated button for this purpose.

As an alternative to changing the current editor behavior, you might want to consider post-processing the HTML to optimize it for specific devices/clients.

The <u> might actually be influenced by the rich text format area settings. But if I remember correctly, it might change other markup as well.

So +1 to the idea. Out of curiosity, is your use case for rich text displayed on a web page? Or is the underline meant to be seen in other formats (document, print, etc.)?

I had the same thought, that <u> was indeed deprecated. But now I need to find uses for the phrase, Unarticulated Annotation. :-)

at the very least, it should be configurable. 

I see it was deprecated and reinstalated - but reinstated for a specific usage sceario - which isn't purely to present something as underlined... developer.mozilla.org/.../u