Thanks for the replies so far. The common thread is protection of the TD in case of other failures. I was thinking along the same lines, but also thought that one single other failure could do no harm, except perhaps a way too high +15V.
For instance a missing -12.2V would shut down U10 and U20, having the same effect as states 1 and 10-16 in normal operation. Then the TD current supply via R97+R98 would permanently be present while the TD is in the low state. That alone would do no harm I think.
I was also thinking of an interrupted R109 and hence missing -5.1V, -3V and -2.5V. That would have more or less the same effect as a missing -12.2V.
In case the zener VR109 dies (to open circuit) U10 and U20 will be destroyed but Q32 doesn't mind.