Has anyone measured what the offset for Bing imagery is around Fremantle, Western Australia?
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I think the best way to check is to open josm and download only the gps traces (no osm data) for the area and display that with Bing imagery. When you select to realign the imagery you will be shown the offsets as you move the imagery around to get the best alignment.
You will likely also also find the alignment varies across the Fremantle area depending on the individual image being displayed for that area. You may also notice where images are stitched together there may be some inconsistencies.
In my experience Esri imagery is much better aligned but is not as clear as Bing.
As the most popular editor is iD and users seem to prefer Bing and few bother to check the image alignment, I expect that many if not most people just accept that most of the area is aligned to Bing and continue further mapping without adjustment just to keep it consistent.
With newer imagery we may get better alignment and higher resolution and that would be the best time to try to re-align large areas to fit the data more accurately to the real position.
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@Frederik are there specific parts of Fremantle that you’re wondering about? I’ve noticed that the change in Bing alignment a couple of years ago as resulted in some areas being quite out of alignment.
If there are any places you want groundtruthing done, I’m happy to go and get some GPX traces if that’d help! (I’m located in Freo.)
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Are there ground control points, where you can see if the imagery is correct?
GCP, are used for shooting airplane aerial images to align them.
If you have the coordinates, you can see if bing is correct.
Challenge accepted 
I managed to find 8 survey marks spread around Fremantle that are visible on the aerial imagery. The clearest one is Node: SSM FREMANTLE 68 (11617835912) | OpenStreetMap, which is quite obvious in Bing. The offset at each survey mark is a remarkably consistent 0.45,0.85.
This offset is the one required to align Bing to GDA2020 and you will see that if you apply this the survey marks will line up. However, a lot of the existing mapping was probably done to imagery that was aligned with GDA94. For the mercator projection in Fremantle the required offset to go from GDA2020 back to GDA94 is -1.20,-1.78. From this we can work out that an offset of -0.75,-0.92 will align Bing with GDA94.
On a practical level what is more important is to line up your mapping with what’s already there, unless you’re going to put in the work required to realign everything. Even then, other mappers will probably ignore the offset and just keep on mapping to what they can see.
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Thanks Andrew, and also Sam for the offer!