Sure the Bitcoin protocol defines a fixed limit of 21M bitcoins but Bitcoin's circulating supply has a number of deflationary pressures such as:
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Loss of keys - as adoption of Bitcoins grows we can only expect a increase in the number of Bitcoins lost by new adopters
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Death of the private key holders with no beneficiary management. I expect the number of Bitcoins that will be lost due to a lack of beneficiary management will peak over the next 5 years before better awareness of and solutions for beneficiary management take hold.
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State seizure of Bitcoins from criminal proceeds including tainting of coins. States have seized Bitcoin from criminal proceeds and while some of these can return to circulation those coins that have been tainted by on-chain analysis firms will be more difficult to re-introduce albeit not impossible.
Some estimates put the total number of Bitcoins that have been removed from circulation at around the 3M mark. If we assume a similar rate of removal in this decade we can expect about 5M bitcoins to be removed from circulation by 2030.
I'd love to hear opinions on this subject because I don't think it is discussed enough and the future ramifications for Bitcoin are quite significant.