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I guess it depends on what your definition of deflation is. But if you consider deflation to be a general decline in the money supply, then bitcoin's currency level actually never goes down.
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In the long-term a general decline in the BTC supply seems pretty likely to me.
As soon as the amount of lost coins (people that die with their BTC, lost private keys
and other scenarios) is higher than the amount of BTC that is created as block reward,
we will see exactly this situation where the available BTC supply declines.
Therefore I´d argue that BTC is inflationary in the first few decades and later turns into
a deflationary asset.