Are there any desktop apps that support Fastmail's label implementation? Also, a Fastmail web app bundled in Electron would still be MUCH faster than Thunderbird with its bundled Mozilla components.
However, there is little movement in the desktop email app space anymore, so aside from their web app, I see no other clients supporting this.
Evolution added some nifty features lately (Markdown integration). And allows to use Client-Side-Decoration annd classic menus - usability wise awesome. Thunderbird got this year a complete redesign.
The other part are we as users.