Como

Thanks, Mark.
The addon from CKEditor appears to be incompatible. But I did manage to add the function and a button a few days ago by following the instructions here:

Ah. I just drilled down a bit into one of the non-indexed videos. Console reports: "Whether the page is a playback page for a single video (Video is main content on the page), or hosts additional meaningful content or videos (Video is supplementary content on the page)."
So, it seems - if I understand correctly - that videos which appear within some threads do not fulfill some mystery Google criteria. What is more perplexing is that the number of indexed videos has actually gone down by about 20%.
Anyway, seems to be nothing to do with IPB or anything which is technically rectifiable.

Hi Marc,
I resolved the problem with a test account. For one, I am used to a global setting for disabling animated avatars. And although I had this set up correctly for Primary and post-count groups, I had not set the restriction for Secondary groups. Those Secondary groups are not even in use now, but many members are still assigned to them since migration a few days ago.
Sorry to have wasted your time.