As far as I could tell based on online sources, this runway was always designed to be used in its current southbound takeoff-only configuration. Thus the only situation I could imagine this taxiway being useful is in the case of a rejected takeoff. I couldn't find good information on the internet about the frequency of rejected takeoffs in commercial aircraft, but intuitively it seems they can't be common enough for a whole taxiway to be dedicated them. If turning around after a rejected takeoff is a concern, why couldn't the runway include a runway turnaround area at the south end?
One rather implausible reason I could think of is that the designers wanted an area for the brakes to cool down on an aircraft after a high speed rejected takeoff without having the aircraft occupying the runway during that time, but high speed rejected takeoffs are so rare that I find this very unlikely.
Even if that is the case, the combination of the southernmost part of taxiway Y5 and taxiway Y9 (the short northwest-southeast taxiway near the south end of runway 18 connecting it to taxiway Y5) makes for an adequately large "brake cooling area". After the brakes are cool, the aircraft can backtrack north along the runway, occupying it for a few minutes at most, and making the long northern part of taxiway Y5 seem unnecessary.
If that is not the case, why does taxiway Y9 exist at all? I find the existence of taxiway Y9 even more confusing than the existence of Y5, as the only case I could think of that leads to taxiway Y9 being used is when an aircraft wants to backtrack northward along runway 18, either by reaching the south end of the runway, exiting via taxiway Y5, and reentering the runway via taxiway Y9, or almost reaching the south end of runway 18, exiting via taxiway Y9, and reentering the south end of runway 18 facing northward via taxiway Y5.
The only closely analogous setup to runway 18 at EDDF that I could find is runway 11 at ZUTF in Chengdu, China. (I'd be interested to see more examples, but such things are hard to look for on the internet.) It has a parallel taxiway (taxiway K) to its north. Some sources give the runway numbering as 11/29 but inspection of aerial imagery confirms that the 29 side does not have a painted number, and there are no landing-related markings at all in either direction, so overall it closely resembles the situation at EDDF.
However, taxiway K is in this case connected to runway 11 at around the midpoint and at the far end (I couldn't find a publicly available airport diagram that shows the numbers of these connecting exit taxiways). Looking on aerial imagery, one can see that these exits are marked "NO ENTRY" in the direction of entering the runway and taxiway K itself is marked "NO ENTRY" past the third entry into the runway counting from the west. This seems to align with the theory that this taxiway is only used in a rejected takeoff.
So basically my question is, what are these taxiways actually for? If they are for rejected takeoffs, how often do the get used for that purpose and how is it worth constructing a whole taxiway just to make handling this rather uncommon event slightly more efficient? And specifically for the EDDF example, what's the reasoning behind the weird configuration of taxiways at the southern end?
Edit: I went through historical aerial imagery a bit and saw that in some interval from around 2019-2020 to around 2021-2022, the main northern part of taxiway Y5 was closed. In that case, taxiway Y9 would indeed serve as part of an "enlarged" turnaround area, together with the southernmost part of taxiway Y5, to allow aircraft to backtrack along the runway. But the aerial imagery shows that taxiway Y9 (historically called taxiway Y7) had existed since at least 2000, so this closure of taxiway Y5 can't be the reason it was put in. But is it possible that potential closures of taxiway Y5 was anticipated as a possibility long ago and they put in Y7/Y9 for redundancy?