During a seminar discussion of the American south in the post-Civil War era, saying that Federal forces should have remained in place and forcibly reconstructed the former Confederacy at the point of a bayonet rather than the Corrupt Bargain of 1876;

Saying that Jefferson Davis, Robert E. Lee, etc. were traitors against the United States government and should have recieved full treason trials;

Being told my idea for a paper was "too niche" when I wanted to investigate the story of Albert Cashier, who is - I would argue - the first fully, formally documented transgender military veteran in U.S. history that we know of so far;

Was informed - point blank - by a fellow student in front of a professor that I was "a terrible historian" for "being so virulently against the South, you can't do your job as a historian with any sort of objectivity. You need to separate yourself from your subject." - and the professor said nothing.

I don't know about "too modern", but some of these definitely come across more as your own personal opinions or interpretations that may be irrelevant to the actual history. For example, "Jefferson Davis and Robert E. Lee should have received full treason trials", which comes across more as a personal opinion than reflecting on what actually happened in history.

To contrast a topic I'm more familiar with, as a disabled amateur historian who has thoroughly studied the controversy over Hans Asperger with the release of Edith Sheffer's Asperger's Children in 2018, a common sentiment you'll frequently find online is, "Hans Asperger was a Nazi, and deserved to be tried for his complicity in the Aktion T4 euthanasia program." There are also false or exaggerated claims often made about Asperger in relation to his claimed culpability and crimes - often based on poor research of the topic - but, in my experience, the often circular conversations such sentiments provoke don't really help autistic people, nor do they really do anything to combat Asperger's primary sin - eugenics - in the modern day and era.

If anything, from what I have seen, such arguments about the "what ifs" and "what-should-have-happeneds" in relation to Hans Asperger, and the community's almost obsessive and pedantic focus on the topic, has actually taken away from - or harmed - real discussions about how historical eugenics has evolved into today's ableist attitudes, as well as the fact that Asperger alone was not wholly responsible for this. Indeed, Lorna Wing - a long-time and highly-respected psychologist who was a dominant figure in the field for decades - was largely responsible for creating "Asperger's Syndrome", and she was the one to elevate Asperger as a historical figure of interest to begin with. Prior to Wing, Asperger's work was obscure and unimportant.

Given this, as well as my other points, I think an argument about the punishments that Davis and Lee "should have received" belong more so on subreddits like r/HistoryWhatIf, as opposed to r/AskHistorians, or history seminars. One could spend all day debating about the punishment (or lack thereof) of Confederate leaders and generals, or the punishment (or lack thereof) of Hans Asperger, but I think the most important points is to present the facts of what happened in history - and why, at the time, these figures were not punished for their complicity.

As for Albert Cashier being transgender, I can understand why some historians might claim your assertation about Cashier being transgender is "presentism". Often times, especially when it comes to the topic of LGBTQA+ people in history, what we will see are many people imposing modern-day ideas about what it means to be LGBTQA+ onto historical figures who might have been considered LGBTQA+, if they were alive today. Again, to use an example I'm more familiar with, "Isaac Newton was gay"; or, more so for me, "Isaac Newton was most likely asexual".

However, it's important to keep one's biases in check, even if we may personally relate to a historical figure because we ourselves identify as LGBTQA+. The people of the past - the ancient Greeks especially, as another common example, as people often claim "Achilles and Patroclus were gay" - often had much different views about homosexuality, homosexual acts, and other LGBTQA+-related actions than we do today. Due to this, some historians are more careful not to project their own, modern ideas of "LGBTQA+ identity" onto historical figures; especially since, as often pointed out, in many circumstances, we don't know all of the facts or details.

My opinion about this is also the same for "[X historical figure] was autistic / neurodiverse / etc." Many people who are completely unfamiliar with, and untrained in, the field of history seem to enjoy retroactively armchair diagnosing historical figures because they feel that it adds to "representation in history", or to "claim" them, but I personally feel that this practice does more harm than good. History is all about facts; and, unfortunately, in the vast majority of some of these cases, we simply don't have all of the facts when it comes to both identity and disability.

That doesn't preclude or prevent someone for making a case for "I think that this historical figure would be considered [insert modern term here] today", but these arguments, in my view, fall more into the territory of "theoretical history", or personal opinion, than historical fact.