user24556: Where can one find statistics on how many nodes are running older Bitcoin Core versions or alternative clients?

user2306: Yes.
Go to https://getaddr.bitnodes.io/nodes/, click on the "User Agents" tab.
At time of writing:
1   /Satoshi:0.11.0/    2119 (35.22%)
2   /Satoshi:0.10.2/    1416 (23.53%)
3   /Satoshi:0.10.0/    437 (7.26%)
4   /Satoshi:0.10.1/    418 (6.95%)
5   /Satoshi:0.9.3/     352 (5.85%)

(You can also click on "more.")

24556: <p>Where can one find statistics on how many nodes are running older Bitcoin Core versions or alternative clients?</p>
 2306: <p>Yes.</p>

<p>Go to <a href="https://getaddr.bitnodes.io/nodes/" rel="nofollow">https://getaddr.bitnodes.io/nodes/</a>, click on the "User Agents" tab.</p>

<p>At time of writing:</p>

<pre><code>1   /Satoshi:0.11.0/    2119 (35.22%)
2   /Satoshi:0.10.2/    1416 (23.53%)
3   /Satoshi:0.10.0/    437 (7.26%)
4   /Satoshi:0.10.1/    418 (6.95%)
5   /Satoshi:0.9.3/     352 (5.85%)
</code></pre>

<p>(You can also click on "more.")</p>
