Context

Core Gutenberg doesn't have an option for underline, but considering the usage data, there is some value in adding it to the style options alongside bold, italic, and strikethrough.

In Aztec, underline is consistently in the top 10, and its usage is roughly double that of strikethrough.

Proposal

Add underline button to Quick Toolbar on text-based blocks such as Heading, Paragraph. I would add this right before the strikethrough option.

Either of these icons could be used:

• Dashicons: https://developer.wordpress.org/resource/dashicons/#editor-underline
• Material: https://material.io/tools/icons/?icon=format_underlined