Unfortunately, it looks like there was a regression that broke this approach somewhere between an earlier beta release of v2 and the new stable release.
From what I can tell, Silverbullet is still serving the static font files, but the browser seems to receive them with the text/html MIME type rather than font/woff2 (I think). I have filed a report on the GitHub issue tracker; please see there for details.
@MrMugame points out that this was a change introduced in #1507. To reference static files in your space, add .fs/ to the start of the URL path (in other words, src: url('/.fs/fonts/...');). Do not create a .fs/ directory on disk in the SilverBullet space itself.
Learn more in the “File system” section of the HTTP API docs.