Maybe we're talking degree or duration, but I think there have been more than a few lefty mashers just in recent O's history.
Steve Pearce, Danny Valencia, Joey Rickard (mash is too strong, but useful vs lefties compared to useless vs righties), Hanser Alberto.
Certainly agree that generally right handed bats have fewer struggles with RHP than left-handed bats do with LHP, so maybe the "lefty masher" title is just bestowed on those guys that end up filling in for the lefties that can't hit LHP.
I've done research on this for DFS. And Derek Carty has done even more. He insists lefty-mashing doesn't exist AT ALL. I think there are some rare examples. Valencia might be one of them if memory serves. I know Pearce had the reputation, but those appear to have been small sample artifacts.
There are certain pitch shapes that perform very poorly in the platoon. So, if you have a guy like Hanser where the scouting report basically says "throw breaking balls," and the lefties he happens to face mostly throw sweepers (very bad platoon splits), then they're set up to fail. One reason we have to reach back in time for our better examples is that teams have gained a better understanding of these factors and can mitigate them more effectively.
Check out the top 50 qualified hitters against LHP: https://www.fangraph...=17,d&page=1_50
Who's an actual lefty masher among them? Austin Slater maybe? Even that's purely based on past performance. He has flat splits this year.