ltdr - game decision with no explainable reason. Hopefully it will be available on other ships with F2P console.
Janeway's blatant disregard for temporal causality (a huge waste of a season finale if you as me) makes this one a hard one to figure out. There are really two things you have to consider.
Did the Borg adapt to the technology? The comments from Janeway in the episode seem to indicate they can/will. This was why she pushed for urgency.
Did the Borg actually get the technology from Janeway at the end? This is entirely based in the assumption that Janeway's virus didn't also kill all the Borg/Borg Queen/stop them from transferring the info to all other Borg. (This, of course, seems to contradict what Admiral Janeway was trying to accomplish but whatever.)
The questions above will create three chains of thought, each with their own unanswered questions.
The Borg did eventually adapt the technology. This means it would only be useful in limited quantities (i.e. one or two shots with a phaser frequency) so it could continue to be useful in emergencies only. However, that doesn't explain why this shielding wouldn't be valuable against other species, although that is also up to speculation since it was only ever applied against the Borg and a fellow Starfleet vessel. Also, I will point out that if it were described as limited use, why couldn't you "press the button and hold it down" for longer periods of time if you needed to? Surely Starfleet engineers didn't build a sub routine to specifically thwart captains from using it in extenuating circumstances.
The Borg were able to get the technology and built a countermeasure. However, this answer poses a number of problems to consider. For example, I know how to drive a car, does that mean assimilating my knowledge suddenly teaches the Borg how to build one? Second, if you were to obtain secret informations about my countries defensive abilities, an my defense has been successful in the past, would I suddenly stop applying this to all of my ships but leave it to be used only by my elite scientists? Third, as with modern tech trends, if my enemy developed a countermeasure, why would I completely abandon the technology instead of trying to improve it? (OMG enough disruptor hits will bring down shields. Oh well it was worth a try, lets scrap it completely and go back to the drawing board).
Janeway's temporal stupidity doesn't actually affect anything. Maybe the tech is just really expensive at this time, requires a lot of power, [insert reason]. The entire argument is based on the assumption that whatever would counter the tech wouldn't be available until some other invention that hasn't come out yet. Either way, we see it applied to a shuttle so good luck explaining that.
In the end, it was just a stupid game decision to make this available on only one ship. Perhaps when F2P comes out, they will make it a console that can be used on any ship. But an in-universe explanation to this, to me option 1 seems to have the most likely outcome based on how I interpreted the Voyager finale.
it's under "Engine/Propulsion Unit (Cont'd)" under point number 31 "Heat Dissipation Vents"
the point leads to the leading "edge" of the warp nacelle pylons.
The dark area has the number 31 attached to it so those would be the sections of the ship serving the exact same purpose as those on the good'ol shuttle.
Could be replicated or stored in a transporter buffer, I suppose, even though we don't see the effect there.
My personal preference is that it involves forming a tesseract around the ship. The ship itselfisn't IN a tesseract but maybe there's a hollow-hull-shaped tesseract around the ship.
Possibly, though I think this technology is closer to StarTreknobable than explained rationally.
Looking at how effective and dense the ablative plates are ingenre I feel that Plottechnobable was more the culprit.