To be more specific:
Hmmm, I should have specified: I'm not exactly talking about trained historians that happen to not be employed in academia at the moment (after all, there's already tons of those sorts of people contributing, communicating, and interpreting regularly right here! And one of my favorite books on history was written by a law professor):
I'm talking about not formally-trained amateurs leveraging their auxiliary or non-scholarly skills, time and attention for the benefit of what historians are already working on. Like how thousands of amateur astronomers point their telescopes at stuff that might be interesting and alert the astronomy community about interesting stuff they've found (they also comb over tons of data that human brains are good at finding patterns in that existing computer programs might not be - that's part of what led to the interesting observations of KIC 8462852/Tabby's Star/Boyajin's Star).
The Transcribathon sound similar to what you describe. Essentially a project to digitise masses of diaries and records from initially the FWW but subsequently other periods too. It’s awesome and is allowing historians to analyse this source material far faster than otherwise possible.
There are “unaffiliated scholars” (read: not employed by any university) that do operate as “serious” historians (as opposed to, say, popular historians). One that comes to mind is Robert Strassler, who is perhaps best known for his critical edition of Thucydides’ The History of the Peloponnesian War.
Ill start with a caveat that I'm not sure if it is exactly what you meant, but it certainly fits in the spirit. With that being said, I think a good example of this is the Tod's Workshop YouTube channel.
The eponymous Tod is a blacksmith who, among other things, makes weapons and armor for movies. He started a YouTube channel "testing" out different weapons. He gained some notoriety to the point where started doing some more serious stuff. Not too long before COVID he did a video where got together with a real genuine historian, a fletcher, and a guy who can shoot a 160pound longbow, and they tested whether or not a longbow arrow could pierce a roughly Agincourt/1415-appropriate breastplate. Video link is here: https://youtu.be/DBxdTkddHaE
Then during COVID he did a bunch of other follow up videos testing various armors (fabric, mail, etc.) and trying to address comments/questions that came up during the first video.
Finally just last week he dropped another series of videos where he got the gang back together, plus another armorer, and they tested the arrows out on a full suit of armor (breastplate, helmet, mail, the whole works). Video link to the main one is here: https://youtu.be/ds-Ev5msyzo
Obviously they're not using huge sample sizes and they have to make a lot of assumptions about what the right gear is, but they certainly found out more than you could just by reading accounts of Agincourt. The whole exercise was done with funding from YouTube content and 4/5 people in the video are not trained historians (the 5th wrote a PhD thesis on 15th century armor so he's clearly a trained academic).
I don't know that what they're doing is going to upend medieval scholarship but it definitely creates a lot of engagement, allows crowdsourcing both funds and ideas, and incorporates skills set like smiting, fletching, and bow shooting that most historians don't have.
Hope that fits the spirit of your question if not the letter. I know it's just one example but I thought it was too spot on not to share.
Trove is an online platform for newspapers etc held by the National Library of Australia. There are lots of amateurs helping transcribe texts. Trove does it automatically and there can be heaps of mistakes and gaps in the older papers. You can jump right in and get started.
This sounds like what local historical societies do. Just about every town in the US has a Historical Society where they collect items like land deeds showing how the local shopping mall was once someone's diary farm and similar. I don't believethey are professional academics, more liely they are retirees or other volunteers that want to contribute in a small way.
No doubt, the history collected by these local societies won't individually transform understanding of whatever period, but collectively, over large areas and with large datasets, they may provide historical info on trends, demographics, etc.