Hello Matthew,
Photo Mechanic 6 does have an autocomplete for the Keywords field as well as the other IPTC dialog fields. The Autocomplete settings can be accessed from the Edit menu -> Settings -> Set Autocomplete. I have my Autocomplete settings to only offer suggestions from the field's popup list. See my screen shot : W10_PM6_Autocomplete_Keywords-1.jpg. I have attached a screen shot of the Metadata (IPTC) Info dialog with the Keywords field showing 3 suggestions after I entered an A in the Keywords field. See : W10_PM6_Keywords_Autocomplete-1.jpg.
If you are working in either the Keywords panel, Edit Keywords panel or the Structured Keywords panel, then there isn't any option for the Autocomplete to offer any suggestions, even if the Autocomplete is set to use a text file as the source.
--Bob