Ngl I can't see it ever not having been an insult, since it's an absolute non-answer.
"Git gud" is not an insult except maybe implicitly. It's simultaneously a troll answer to any comment like "how do I beat this" or "this is too hard" and actual advice because, in a Souls game, most of the time the answer to the question of "how do I beat this" will boil down to "learn the dodge timing and improve your basic gameplay skills", which itself boils down to "just keep at it until you get better at the game".
Sure, sometimes there's a strategy or trick that actually makes a fight easier, and sometimes there's a build or equipment combo that can cheese things up to a point or make an otherwise difficult fight very easy, but those are hardly universally useful because sometimes a boss just has a mechanic or moveset that shits on your build.
Go to any Souls-like boss discussion and there will always be a person saying "X boss was almost impossible" and another guy will reply with something along the lines of "X boss? I beat him in the first try lmao". Then there's the people who agree the boss is difficult, or tell you they also had a hard time against it and give them advice that eventually boils down to "learn to dodge this", "learn to hit this weakpoint" and "just take it slow and be methodical", which again will boil down to "get better at doing the basic gameplay stuff" or in other words, "git gud".
It's definitely a non-answer, but also the most useful advice you can give a player, because if you learn the fights and stick to the basics you can beat just about any boss in a Souls game without armor, buffs, spells or fancy weapons. Sekiro is the best example because while there's a lot of fancy abilities and stuff you can use to spice up the gameplay or make a fight faster, you will need to learn how to Deflect, dodge Perilous Attacks, and attack in between the boss' attack strings, and if you can do those three things well enough they'll be enough to beat everything in the game, including the superbosses.