Is extra-planetary phosphine actually a new discovery? For all the buzz on Venus’s phosphine, I’m curious as to what sets it apart from the findings of phosphine on Jupiter,  Titan, etc.  There are several papers on NASA.gov dating to 1979 and more recent on the subject.   Per request in comments:  https://astrobiology.nasa.gov/nai/annual-reports/2010/jpl-titan/the-commonality-of-life-in-the-universe/index.html
Why is this latest finding of Venusian phosphine a special case?
You're correct; extraterrestrial phosphine is not actually a new discovery. As you said, we've known for decades (see e.g. Ridgway et al. 1976) that phosphine can and does exist in some gas giant atmospheres, including Jupiter's. However, detailed modeling has been done of the associated photochemistry beginning at around the same time, and as far as I'm aware, the important thing is that the observations aren't in conflict with the models.
The recent claim of phosphine detection (Greaves et al. 2020) went beyond all of that because as it indicated that phosphine appeared to exist in higher concentrations in the Venusian atmosphere than could be produced by known processes. From what I can tell, the group behind the detection was fairly exhaustive in their attempts to account for the concentrations by other means - lightning, known photochemical processes, meteorites, etc. All of them fall short by quite a lot, by orders of magnitude in most cases.
That's the key: It's not that phosphine was claimed to be detected, but that too much phosphine appeared to be detected. All that said, doubt has been cast on the results. Recent work (Snellen et al. 2020) found that the method used by Greaves et al. involving a 12th-order polynomial can also lead to numerous false positives when applied to other spectral features near 267 GHz, the frequency of the claimed detection. The reanalysis of the data implies a feature detected at a signal-to-noise ratio of 2, which is certainly too low to imply a statistically significant result.

(As an aside: the paper you cite (Pasek et al. 2011) isn't an observational paper - that is, it doesn't report an actual discovery of phosphine on Titan. Instead, it describes theoretical modeling and some laboratory work involving phosphorus chemistry. It's one thing to say that &quot;Hey, here's how phosphine might participate in chemical cycles in this environment&quot; and another to actually confirm the existence of that compound.)