I created bug report EBR-230767 about this, but I’ll post it here too:
Some skillbooks are no longer being sold by NPCs. These are the ones that I have noticed are affected:
- Mining Barge
- Exhumers
- Deep Core Mining
- Industrial Reconfiguration
- Capital Industrial Reconfiguration
- Gallente Industrial
- Minmatar Industrial
- Capital Industrial Ships
- Mercoxit Ore Processing
Looking at the price history for these items, it looks like the issue began around December 7-10.
It looks like Version 19.11 released on December 8. That was the update where the ore processing skills were consolidated. It could have something to do with that.