On the morning of the 3rd of November, JWS25 of the Old Digibox Discord Server noticed that his Pace TDS850NB PVR4 Sky+HD box, which was on 5.10.jHD (a rare version), had updated to the final 2017 version of R010 and that his recordings were wiped. The same happened for Chelmsford IT's Pace, which also received this update, but the recordings were unaffected.
According to Sky's Live Software in the Field report, as of August 2020, Sky had pushed these updates out to Pace and Samsung PVR4 units as far back as February 2017. The reasons as to why the units didn't update until now are unknown, though it could have been a fault in the Live Software stream that Sky fixed.
This suggests that it won't be safe to keep PVR4 units plugged in that are on an older version that you want to keep, as we never know when Sky will deploy updates to them again.
Theories in the Discord server suggest that the R010 update deployment for PVR4 units on the Live Software stream will only last a month, but evidence is lacking. For now, to quote user sabianmateinnit, "it is simply too much of a risk to plug in historic PVR4 units."