What is the current recommended Eclipse version and plugin update site?

There is nothing in particular to Eclipse that'd make one version a non-recommended version. There's no specific version of Eclipse categorized as recommended. Any of the recent versions will be enough.

I've been using Neon without any problems.

Link to update site forum.appian.com/.../Appian_Plug-in_Creator_for_Eclipse.html

Keep in mind that the JARs are from Appian 6.6.0 and to be used as a reference. Should your version be newer you need to replace the JARs with the ones from your version of Appian to benefit from all API changes that might be newer than 6.6.0

There is nothing in particular to Eclipse that'd make one version a non-recommended version. There's no specific version of Eclipse categorized as recommended. Any of the recent versions will be enough.

I've been using Neon without any problems.

Link to update site forum.appian.com/.../Appian_Plug-in_Creator_for_Eclipse.html

Keep in mind that the JARs are from Appian 6.6.0 and to be used as a reference. Should your version be newer you need to replace the JARs with the ones from your version of Appian to benefit from all API changes that might be newer than 6.6.0