This wikipedia article quoting a standard formalised in the late 90's, and only beginning to gain adoption from that point on, largely supports my statement.
The Wikipedia article lists things that always used the base 10 definition. It has examples of disks from the 1970s. And it mentions a 3rd definition of megabyte used since the 1980s.
[1] https://en.wikipedia.org/wiki/Byte#Units_based_on_powers_of_...