Go to File / Options / Data and check the “From Text (Legacy)” option. Then select OK.
Now then launch the legacy Text Importer in Excel by going to Data / Get Data / Legacy Wizards / From Text (Legacy).
The legacy text importer still seems to be quite a bit easier to use than the new Text Data Sources available in the latest version of Office 365, so our recommendation would be to keep using it.
Alternatively, if you go to File / Open in Excel and select a text file, the legacy text importer wizard will launch by default, regardless of whether you have enabled it in the Options menu.