Thank You For This Thread.
Today I did a manual install of the ident_switch plugin on a new Roundcube 1.3.4 installation running the Larry skin, initially I was not getting the account selection list in the top right of the screen, after finding this thread I modified the plugin Javascript files as suggested, replacing size() with length , and now it works fine, the account selection list works and switches accounts successfully, I also installed the identity_smtp plugin by doing a git clone of it into the plugins directory and that plugin works nicely as well, so I can switch accounts and send from the appropriate smtp server for the account.
These plugins do work under Roundcube 1.3.4. I am running PHP 7.2 under Apache 2.4.6 on Centos 7.4
Colin