To expand a little on what Taladris said in their comment, and to be a frame challenge, the main object of the data connection being a large pointed spike might be to startle the viewer. Of course, this is an "out of universe reason", but the question isn't exactly an "in universe" question anyway.
IIRC, the first time Robocop extends his data spike, he startles the nearby cop. To me, it's obvious this was done for dramatic effect. If the cop was as tech savvy as he should have been in that situation, it shouldn't have surprised him by recognizing it for what it was. It also happens to be located where it can emulate a lewd hand gesture, which isn't at all necessary for a data port.
Later on, Robocop uses it to kill a badguy. This, again, is more of a dramatic reason for having it as a spike than any other shape. Robocop could easily have grabbed the guy's throat and ripped it out, instead of spiking him in that same scene.
R2D2 was a maintenance or copilot droid, so he was compact without arms or tentacles, so he needed something to physically connect to the machines he was fixing or flying.
Since it's easier to connect 2 things when the male end is pointed, it makes sense this isn't a square end. NASA uses a similar technique when docking space craft. The tapered end allows you to get "close enough" and then it seats itself. In an emergency situation, the C3PO robot wouldn't necessarily have the time or the steadiness (think loss of artificial gravity) to spend 30 seconds to get 100% lined up with a data port.
There's also the aspect that this port is rotated while in use, so something round would make easier insertion, rather than a rectangle that doesn't allow being off by even a handful of degrees, as in the dreaded USB connector which live in the 4th dimension.
Vampire taps are a thing, but they are generally enclosed as well as small. What they do is puncture the insulation around a cable as well as slightly embed in the wiring. This is so a technician can tap into the wire without having to cut and splice the original wire, and was considered a permanent or semi-permanent situation. Removing the tap was allowed, but precautions had to be made to prevent the wire from being further damaged after removal, such as with spills or bends.
Vampire Tap Wiki page
I can't get a good read on when vampire taps were first used, but it appears to be the 70's, so would predate both Star Wars and Robocop.
I think I've made a case for the data connector to not really have any realistic precursor in reality, but it's either for dramatic effect (Robocop) or it's a practical application of technology (Star Wars). Vampire taps seem to have existed before both movies, but it's unlikely that anyone outside of geek/nerd culture would know about them at that point in history, and they aren't an obvious precursor to either movie device.