What I do, is to make a Calculated field, of typ nvarchar, that returns “YB” if the boolean field is true, and “NB” if it is false.
Then in Reports and Dashboards, you can filter on that field, with a filter type CONTAINS. Then the user can Enter a ‘Y’ to get records where the boolean field is true, ‘N’ for records where it is false, and ‘B’ for both.
Technically you don’t even need to Enter ‘B’, as leaving it blank also returns all records.