Has anyone used the "Auto mark unused for trade" from the Inventory screen and, if so, how were your results?
Accurate? Nothing messed up in inventory/tradeable status/decks?

I see the warning prompt clarifying what will happen, which is what I want to ultimately do...just wondering if the outcome was a good one for those that used it before I pull the trigger on this. :)

I have! It all works out. Take a backup if you feel nervous!

I have some cards that I don't want to trade even if they are not in a deck. I have a "Do not trade" tag on these, and what I do is turn the Auto-Mark for Trade feature on, and then I turn it back off again and go filter the trade list by that tag and remove them from the list.

Interesting, and good to hear! Is the "Auto mark used for Trade" function working in a way where, say I do this...then every time I create or dismantle, say, and EDH deck or something, I can just hit "Auto mark used for Trade" again and it will just set my tradeable list correctly over and over again (so I don't have to individually mark cards tradeable on the decklist prior to me deleting the deck entirely)?

I was a bit skittish on this because my buddy was telling me I tried this function about 4-5 years ago and it severely messed something up in my entire inventory/tradeables/whatever...but I can't remember the specifics. However, I guess I could also take your advice and create a backup of everything ahead of time just in case.