They got rid of the 'most relevant' section recently probably due to people taking advantage of system by using the same keywords in the title and the keywords section.

And it looks like your assessment is correct. Most relevant is a combination of most popular and relevant, but mostly popular.

More useless fiddling by shutterstock to make themselves appear fresh and relevant

Spot-on!!

Not sure if this will affect any sale. But I really like the envato filters option which lets you refine the search with date added and popularity in specific period.

Introducing this feature in ss might be very useful for buyers.

Setting "Fresh content" as default instead of "Most relevant" is what is needed to give new content a chance and motivate us to upload good content.

But then they ought to review new content properly.

words of wisdom for SS:  'If you paint a piece of dog crap with gold paint it still is dog crap'

BUT! it's in GOLD PAINT!!!

don't forget that!

When I search on Shutterstock now I see "Most relevant" and "Fresh content".

"Fresh content" looks exactly like "New".

Not sure what "Most relevant" is meant to be, looks like a mix of new and popular to me.

Did they just change names, or more?

Since I have no recorded searches or screen captures, and from looking, I see no change, I'd suspect they changed the names. Yes Fresh Content is just New with a new name. Most Relevant avoids the impression that what's showing is Best Match, because it's not. 

Someone might ask Most Relevant to what?