We don't know.
On the one hand, Janeway is almost pathetically eager to follow the Temporal Prime Directive and return to the Delta Quadrant with Braxton (instead of just performing a few near-lightspeed trips around the nearest star to eat up a couple of hundred years). On the other hand, she doesn't think twice about holding onto the mobile emitter, a piece of 29th Century tech so advanced that it could (and nearly does) result in a dramatic technical advantage to any race that lays its hands on it.
In the absence of any info from the episode, the most likely outcome seems to be that they kept it for study, but that this resulted in no dramatic advances (or at least none worthy of mention).
In the novel 'Star Trek: Department of Temporal Investigations: Watching the Clock', Janeway has a brief conversation about the mobile emitter. It would seem that while 29th Century tech is advanced, the underlying principles aren't all that and a bag of chips.
“On Stardate 50312, you allowed your Emergency Medical Hologram to
keep a mobile holo-emitter based on twenty-ninth-century technology,
rather than confiscating and destroying that technology.”
“The Doctor needed that mobility to be able to do his job at peak
efficiency. I couldn’t risk having my ship’s only medical officer
trapped in sickbay indefinitely.”
“And didn’t you think about the consequences to Federation
technological progress once you got back?”
“Frequently. But I studied the device. Whatever century it came from,
its operating principles weren’t too far ahead of the current state of
the art. I expected that by the time we got home, probably decades in
the future, the technology would already have caught up.”