(I have not tried Thunderbird 68 yet, but have tried a fairly recent one last month, unknown version though)
Favorite interface for mail clients are Airmail (macOS), Mailbird (Windows), geary (Linux).
The thing they have in common is unified inbox, merging of threads, controlling signal-noise.
If thunderbird took that UX approach, had undo send, it'd be indispensable.
There are also small things, like google apps 2FA working out of the box, searching email, that for some reason haven't worked as well for me w/ thunderbird yet. I'm not sure if searching gmail is doing it through their API, or by downloading all mail, but I've had issues with that.
As usual, I came here to look for recommendations :) I have been enjoying Mailspring[0], an OSS fork of Nylas Mail (RIP). I came across it while looking for the closest thing to Google Inbox (RIP also). Mailspring is cross-platform.
It's free, but not open source.
Or are you referring to the cloud aspects of Mailspring, like read receipts etc?