Q How can I keep old versions of key applications on my iMac so that I can still run those which are, for example, incompatible with Yosemite or El Capitan. Could I have two bootable drives with different versions of OS X?
A Yes you can still do this.
It normally works best if the two versions of OS X are only 0.1 apart, rather than 0.2 or more, but in principle you should still be able to get the Startup Disk pane in System Preferences to recognise both and allow you to choose between them.
You will need to be disciplined as to what you keep on each disk, or you could end up installing incompatible apps or – even worse – extensions and the like on the wrong disk.
Updated from the original, which was first published in MacUser volume 28 issue 16, 2012.