I guess the only minute difference would be how we use them. 'over the past decade' means that it happened repeatedly, more than once, or. In a decade = in 10 years time this has no specific date.
The Iconic Evolution Of Dolly Parton Hair A Journey Through Glamour
In the/this decade = in these 10 years this implies you're talking about the current decade.
I mean we can say two decades or three decades to.
