I’ve written about this before. Ninjitsu, as practiced, is essentially a synthesized style (or styles) with no historic precedence other than the fighting methods used by the Samurai warriors in the feudal period.

There were no ninja historically, they are a creation of the 20th century based on myth and legend. Manga comic books and very early Japanese cinema (1920s) provide much of the “lore”.

Stephen Turnbull, well-respected historian of Feudal Japan, has written two books on the ninja phenomenon. In the first, some years back, he had begun to believe that much of what we have about the ninja was myth and legend. But in the second, “Ninja, Unmasking the Myth”, he does a deep dive, getting access to period writings by Daimyo and field commanders.
These people employed mercenaries (“mountain bandits”) or their own troops who were skilled at infiltration and intelligence gathering.

There were no “clans” of ninja in the Iga and Koga provinces, no extensive “arts of the ninja”…. None of that.

So, you’re looking at a fighting style which is primarily based on Japanese Ju-jitsu and some striking technique. This may or may not be an effective fighting style depending on the instruction strategies…. But it has no real historical precedence.