font-display: swap; impacts the block and swap time when waiting for a font to load, but we’re including the font from the backend (we have a separate discourse-fonts gem — doing it this way eliminates making a call to Google and being tracked).
So because fonts are included as part of the Discourse application on initial load, I don’t think we’d see any benefit because fonts aren’t holding up rendering like they would be if we were calling out to Google (or another CDN) in our CSS.
Using font-display: swap; is still probably good advice for anyone adding custom fonts from a third-party CDN in a theme though.