The interview process started well with a recruiter call.

It then progressed to a technical interview, where I was asked about implementing a cache class with concurrency and thread safety. This also went well.

After that, I was told I had passed to the next stage, but shortly after, I was informed that the position was no longer available and had become a frontend-focused role.

I stated that I could also do frontend work. However, I was subsequently ghosted.

As other software engineers have noted, I don't believe they actually have any positions available. It seems they are "guinea pigging" people to gather data for some process. Multiple individuals have shared similar experiences on Glassdoor.

Implement a Cache class with certain Big(O) constraints and thread safety.