In this sentence
... but I’d rather focus on the future rather than what's been
we see "been" at the end, I would never think to make it that way, I would put "was".
... but I’d rather focus on the future rather than what was
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It's what's been because it's using the present perfect tense.
what's been is the same as what has been.
has been is the present perfect tense, which is made with to have + the past participle of a verb, and the past participle of to be is been. What has was doesn't make any sense, like DrMoishe Pippik says, because was is not the past participle of to be, and "has was" isn't correct conjugation.
What might be confusing you is that you could also say
I’d rather focus on the future rather than what was.
That is using the simple past was rather than the present perfect has been.