Currently, people can select some text in a message and hit reply to start composing a message that is a) in reply to the message with the selected text and b) quoting only the selected text. This is currently not very discoverable however, and hence could have some affordance added to indicate it is a thing that exists.

One option would simply be to add a context menu item for the selection, but that's also pretty undiscoverable for normal people.

Another would be to display a popover on selection, that includes a "Reply quoting" item (may better worded), and also say items for other current undisoverable actions such as Find and Copy. Examples of this are Bijiben's selection popover and similar on mobile apps. This might get annoying for people though who offhandedly select text in messages.

Another might be to display a new action bar at the bottom of the message, or add a reply quoted button to the message actions at the top of the toolbar, but these may not be visible unless anchored somehow.

The popover is probably the most discoverable (just select some text), and unlike the action bar/message action button directly connects the selected text to the action - you can't miss it.