@anon36505037 is fairly new to SmartThings and lives in the UK. He is very knowledgeable about the use of in wall micro relays as he has done his whole house that way, but there are very few wall switches available for use in the UK with SmartThings, so I’m not surprised if he hasn’t heard of the use of association on the SmartThings platform before.
That said, it is very frequently used in the US, where we have a wide variety of switches, including some which are specifically designed for the use of association to create a virtual three-way between two powered wall switches. In this case, no traveler wires are needed. Several of the most popular switch brands in the US rely on zwave Association for their three ways, including GoControl, Cooper, and Leviton. So you will find many forum discussions about it.
In particular, linear/go control (which are not available in the UK) make a very popular three-way set. Both switches are wired to mains power, but they do not use physical wires to communicate to each other. Instead, they use association to make a wireless connection between the master and the auxiliary.
We even have an FAQ on how to set up this association:
One of the unusual things about smartthings is that it also allows you to set up the equivalent of an association but between any two devices that your hub can talk to, including devices of different protocols or very far away from each other. That’s what the official smartlighting feature can do, Among other things. However, that method does require that the hub be operational as it is acting as the “man in the middle.”
And many community members are using both methods, relying on Z wave association for devices which meet the criteria I listed above so that they will continue to be operational even if the hub is out, and relying on the hub and the smart lighting feature, or other smartapps, for everyday cases that don’t meet those criteria.
(The GE switches, another popular brand, do use physical traveler wires, so again, you just have to check each model that you’re considering.)