Try this one
where datediff(month, datetime_column, getdate()) <= 6
To exclude or filter out future dates
where datediff(month, datetime_column, getdate()) between 0 and 6
This part datediff(month, datetime_column, getdate()) will get the month difference in number of current date and Datetime_Column and will return Rows like:
Result
1
2
3
4
5
6
7
8
9
10
This is Our final condition to get last 6 months data
where result <= 6