Do I need to inform the other company regarding my job offer?

Context: I resigned from Company A in March after signing a job offer to start working in Company B. The pandemic broke out so Company B keeps pushing back my start date. Fast forward to October and I still haven't started working at Company B. They did not rescind the job offer but whenever I ask about my start date, their answers are non-commital, "We'd love to have you once we're fully operational."

I've been working part-time, but when October rolled-in, I decided to start looking for another full-time position. I received a job offer yesterday that I would like to accept.

Do I need to inform Company B? I'd still like to work with them once they're 'fully-operational'.

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