Right now there is a thread on Hacker News about death of Stack Overflow Jobs. As part of the mourning process, people are now compelled to talk about their fond memories about the service. If you're looking to make a tech-oriented job board: this is a great learning opportunity to find what differentiated the service and how to eventually attracted its displaced users.
As an example of a nugget of learning, there are users bemoaning the loss of the Dev Story feature:
It was a perfect fit for SO, and I used the dev story as a CV replacement. Helped me get my last three jobs. I was hoping to find out why they're doing this from the article, but I really didn't, and I don't get it. Oh well. Still not getting a LinkedIn.
Indie Hackers podcast episode #096 How Three Founders Created a Profitable Business That They Haven't Even Launched Yet with Ben Orenstein of Tuple is great example of how the loss of an existing service (ScreenHero in that case) can create a gap in the market.
I personally have no interest in the space, but I hope it helps someone else here!