I can't say for sure when that agitator was gone for good, but I am guessing by 1968, at least in Kenmores. Two reasons for that:
1) In 1968 the gold polyproylene Straight-Vane debuted in any Kenmore that in 1967 had this older version.
2) During the mid 1960s, the bakelite, drive-block agitators began to be replaced by polypropylene versions with molded splines instead of tapered drive blocks. This was the last agitator to be replaced in Kenmore.
I am pretty sure that any black six-vane equipped model in 1967 was either replaced by a 1968 version with the gold plastic agitator, or was carried over and revised with the new agitator.
When the conversion happened from bakelite to 'propylene in the Roto-Swirl, the new agitator looked identical. This one was changed considerably to have more-shallow fins and the lower three "humps" were removed.
This agitator was used by Whirlpool early-on, and was used by Brasstemp as well in Brazil. It may have remained in use there longer, I am not sure.
Gordon