You can tell someone is older or in academics/research when they say it like this.

For most people “data” is a singular set. It’s clearer to say “these sets of data are conclusive”, while if it’s a single study, “this data is conclusive”

Like why you don’t walk into a police station and say “there were murders everywhere” when you’re talking about crows.

This datum is conclusive!

In some languages the word "data" is always plural. It feels natural to say it like that in english, too.