I forgot about it, but I launched a script a few weeks ago, so that I can now give some stats about the images on phereo.
There are 248536 public images and 18180 of them returned an error (bypassing the certificate errors), which is 7.3% or 1 over 13.7 in average [at least during this test period]
I did not test all variants available on the site, only the biggest definition.
The images still available weight about 157GB (only one variant) as far as I can tell, and the metadata probably around 100MB.
Repartition of the losses over time is attached.
The intuition that most errors appeared around early 2019 is confirmed, and almost all of the images published after this moment are apparently lost. :-(