It's getting wider ownership over time so BTC supply is increasingly limited per person/entity interested in it, and the increase in supply is at a decreasing rate over time. This wider ownership and the (ultimately) fixed amount means that it will, even if not monotonically, be a deflationary currency (to the extent that it even is a currency). The rate of new BTC production has to overcome this deflationary pressure, and since there's always less coming out over time the potential inflationary pressure of minting new BTC will become increasingly insignificant.

Any currency with a limited supply will tend to be deflationary over time, though not necessarily experiencing a deflationary spiral depending on the overall economy around it. An active economy will see enough movement of the currency that deflation will, hopefully, be small. But an inactive economy (like bitcoin's) with people hoarding rather than spending will see higher-than-healthy (from an economic perspective, it's great for the hoarders if they can use it later) deflation rates.