Ad-driven skills gap?
I don't buy the explanation of this being vagaries of advertisements, regardless of the job.

Instead, I see it as employers showing rational and predictable flexibility in response to a changing supply of labour. 

At times of plentiful supply, they can reduce recruitment costs by being far more selective at the first stage - filtering by essential requirements in the job ad. Finding good staff can be an onerous process. All the filtering stages after writing the ad are labour-intensive, and the labour is often line-managers and upwards, so not cheap. If I know I can get a decent number of interview candidates even with very stringent essential requirements, I'll do that, and that will reduce the number of applications that I have to process. This reduces my costs without significantly impairing the outcome.

At times of restricted supply, it's more cost-effective to advertise for lower essential requirements. That way, you cast a wider net, get more applicants, and are more likely to find a suitable candidate, without overwhelming the staff who have to read through the applications and process them.

As an occasional recruiter, I've done all this myself.