Now if a Keynesian when using the term 'deflation' means that price appreciation then by golly they are correctly describing BTC by what that word means to THEM. If someone else with this weird idea that these words mean only nominal changes in money supply says BTC is inflating then yes they are also right by what that word means to them. All we have here is a disagreement over what words mean, not the nature of BTC which everyone agrees on undeniable fact, that of growing supply and appreciation in value.
Now I'm going to argue that the so called 'Keynsian' (really everyone excluding a few wackos) definition of inflation/deflation is correct. Why because it actually MEANS something to an economy and an individual, where as the nominal money supply tells us nothing about an economy or what to do as an individual. Real inflation is an incentive to spend, real deflation is an incentive to save, BTC clearly falls under that latter kind of incentive structure (the Austrian and Keynsian would now have an actual disagreement over if this is good or bad). If I make a statement about nominal money supply while ignoring the population of users, the quantity of production and the demand for consumption of those users I've ignored so many CRITICAL factors that I no longer say anything meaningful about the signals being sent, are individuals under an incentive to spend or to save, I haven't said one way or the other unless I make the assumption that ALL the other factors are constant which this narrow definition explicitly excludes if it mean ONLY nominal money supply.
No, I don't think we can all agree about that. We can all agree that the supply of bitcoins is increasing, therefore there is (monetary) inflation. But the price of bitcoins requires knowing other things, which can be guessed at but are unknown. These things include the adoption of bitcoin, the usage vectors of bitcoins, and the amount of credit built on top of the bitcoin economy. We can say that if things continue to go the way they are now going then the price in bitcoins will go down and we can describe it as price deflation, but that relies on events unfolding as you predict them to.
If you look back at the historical price of bitcoins, there have been several upward movements which are correlated with influxes of people to the bitcoin economy, followed by long, slow downward trends. This is because bitcoins are in fact inflating, and without influx of new money the price is pushed down by the increased supply. If you look at the historical price data (it helps to put it on a log chart so you can see the movement a couple years ago), you can see that this effect was more pronounced years ago when bitcoins were inflating at a higher rate.