In EF6 DbFunctions.TruncateTime is used instead of DateTime.Date property because for some reason the later is not supported.
In EF Core the former is not needed simply because DateTime.Date now is recognized and translated correctly.
group events by events.DateTimeFrom.Date into dateGroup
Unfortunately there is no documentation (yet) of what is supported, so as a general rule of thumb, always try the corresponding CLR method/property (if any) and check if it translates to SQL and how.