One can argue when these terms were initially coined. But we do have hard data on when they became prevalent in the public mind. Look at the Google trends for "deplatforming"[1], "no platforming"[2] and "alt right"[3]. "No platforming" had some blips starting in the late 2000s, but begins rising significantly in 2015, "deplatforming" in January of 2016, and "alt right" in august of 2016. There is evidence to the claim that deplatforming (or at least, widespread interesting in deplatofrming or "no platforming") preceded widespread interest in the alt-right.
> The people stay, the community goes. Then, you look at the level of the material in other sub-communities on the site. That is what those studies did, and that is why they demonstrate causality.
Yes, but as I stated multiple times by now the key limitation here is that they only looked at the material on the same site. Site X bans Y (whether in full or in only some subforums). You observe a reduction of Y on the site. That's not evidence that this action reduced Y in society as a whole. There is a causal relationship between deplatforming and reduction of the deplatformed view on said platform. Nobody is disagreeing with that - most people would likely read such a statement and think "no kidding, Sherlock".
For example, pointing to the fact that when Reddit banned racist subreddits racist content on other subreddits were reduced is proof that banning racist subreddits reduced racist content on Reddit. This is not at all surprising, and is something most would call obvious. But to portray this as proof that banning racist subreddits reduces racist content in society as a whole is a very large misrepresentation. This study did not study the impact on society as a whole - only the forum that is carrying out the deplatforming.
And again, I do not attempt to claim the the correlation between the rise of deplatforming and the rise of the alt right is irrefutable proof that the former causes the latter. But claiming that the former helps prevents the latter is not backed up by the evidence we do have.
1. https://trends.google.com/trends/explore?date=all&q=deplatfo...
2. https://trends.google.com/trends/explore?date=all&q=no%20pla...
3. https://trends.google.com/trends/explore?date=all&q=alt%20ri...