It is mandatory for the lookup file to have this for an automatic lookup to work.

Source type is a configuration setting used to define which events trigger the automatic lookup, not a required column inside the lookup file itself.

There is no minimum column count requirement for a lookup file; automatic lookups can work with as few as two columns.

A timestamp column is not required in a lookup file; lookup tables are typically static reference data without time-based fields.

An automatic lookup requires an input field - a column in the lookup table that corresponds to a field present in the search events - so Splunk can determine which lookup row to join to each event. Without this matching input field, the automatic lookup has no basis for linking lookup data to the correct events.