Ratings and characteristics are two different things. I think you've mixed up which section is which!
Characteristics are what you can expect under ordinary conditions. You might be using a 1N4148 at logic levels with pull-up resistors and such, and so the characteristic at a couple mA might be relevant, and so they give a figure around there. Notice they might not give full min/typ/max, only the best or worst case, because that's all they'll really guarantee. These diodes do not make great voltage references, for example -- the Vf might differ by 100mV at a given current, between individual diodes.
You can very reasonably assume diode characteristics follow the nonideal model. That is, the Shockley equation for the most part (current is exponential with voltage), but with ESR added in. There's also reverse breakdown (avalanche or failure), and excess leakage (particularly for glass-body types which are slightly photosensitive). That describes the DC behavior. At AC, there is also capacitance and reverse recovery. That's more than a few things in total, but computationally speaking it's not a problem, and this set of models describes everything a full-on SPICE simulator uses -- and does a pretty good job at, all in all. (The main thing missing is forward recovery, which is only applicable to some types and circumstances, and is harder to simulate anyway so it's fair that they left it out.)
So, that's a lot of flavor, but the takeaway is the DC property, current exponential with voltage. This is obeyed down to very low currents indeed, say +/-1nA range for voltages in the +/-100mV (or even less, not sure) range.
Likewise, for currents around nominal (say 1-100mA), you can very reasonably assume a fixed voltage, or a voltage plus resistance. This is where the oft-quoted "0.6V" or "0.7V" comes from. It's actually a segment of an exponential curve, but because current is exponential with voltage, the inverse of that is voltage logarithmic with current, and so the voltage only varies noticeably over huge ratios of current (like ~60mV/10x), and for ordinary currents in that range, well yeah, the voltage is fairly stable.
So, driving your LED matrix, expect to lose about a volt, maybe slightly more if you're driving it >10mA, maybe a bit less if just a few mA. It seems unlikely you'd drive an LED matrix much lower than this, so a noticeably smaller Vf probably won't be encountered.
As for the intended application -- I would instead recommend powering the LEDs and MCU from the adaptor, using a schottky diode each, such as 1N5819 or B140. Don't put diodes around the ISP: the programmer should get exactly the MCU voltage. Schottky have lower voltage drop so you lose less of the adaptor voltage in normal operation. I'd also recommend programming with main power applied anyway, so this is all a non-issue.
This [avoiding ISP diodes] is probably most relevant with official programmer adapters, which have voltage interface circuitry for protection and flexibility, and so need to read the target voltage to set their output voltage level, and input voltage range. They do not power the target (at least, the ones I've used don't; not sure about all of them). Whereas your Arduino I'm guessing, is just powered straight in parallel with the MCU?
Tim
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