Nobody knows about future jobs for Prompt Engineers, but I suppose that such a job should require reading the new techniques and papers about LLM and be able to discover what type of context is better, so categorizing, clustering, A/B testing and some math skills could be good to have for such roles. I

If I was to interview someone for Prompt Engineering at least the candidate should be able to look for information in the web with the least effort, that is you should be able to collect important information with the minimum number of queries.