I’ve thought a lot about this in terms of historically and to the OP’s specific phrasing of “today”, I retired in the early 2000s but I think at least the majority of the structural stuff remains the same. I think the fairest “not looking for atypical situations” answer is Captain (O-3) in the Army and Marines. Anyone up to Brigadier General (O-7) could still end up in combat in situations that would be atypical but not “newsworthy.” Anything above that would, in modern combat, likely be some sort of terrorist attack or something like that, which I think is a whole debate about whether that’s quite the same thing as “on the front lines.” If you want to include that sort of thing there’s no real limit, terrorist blew up part of the Pentagon on 9/11/01 and that building houses people as high in the rank structure as you can expect to find.
I was out before Iraq / Afghanistan, but I distinctly remember at least a couple news article from the early days of the Iraq occupation where O-7s got in slightly hairy situations. I tried to find some of them but haven’t been able to thus far, my memory was that some guys of that rank were regularly traveling around occupied areas to meet with local Iraqi leaders as part of government building operations, and this occasionally put them in a situation where their convoys were in action with insurgent forces.
In WWII there’s a number of people in the O-7 rank who are documented to have fought in direct actions during D-Day. Stephen Ambrose’s book Citizen Soldiers details a few of them (the book covers the Western European theater from D+1 until V-E day. Note that Stephen Ambrose (who was a “pop Historian”) suffered reputational hit near the end of his life due to poorly cited / footnoted work (aka plagiarized) and some factual inaccuracies, but I think the accounts referenced are still fairly well documented.