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was or has been?

He finally takes off from what was probably a boring party.

He finally takes off from what's probably been a boring party.

Which one?

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anonymouswas or has been?He finally takes off from what was probably a boring party.He finally takes off from what's probably been a boring party.Which one?

Both are common. You must choose with a good sense of writing style, which depends on the context and flow of your narrative.