This means that as of the current version of UKanDo (3.7.4). As well as providing the option for users to have UKanDo automatically update on a new release, these changes mean that only the current release of UKanDo and the two immediately prior to it will be allowed to connect to Second Life; older versions will be automatically blocked, requiring users to update them.
The UKanDo blog post announcing these changes reads in full:
UKanDo now uses the Linden Lab® Viewer Version Manager (VVM) system for automatic updates and version management. As a user you have the option to set automatic or manual updates in Preferences-> Setup-> Software updates (by default it is set to Install automatically).
The release candidates tick-box is disabled as there are no plans what-so-ever to use this feature.
VVM is also used to control which versions can or can’t login to Second Life®. The versions which can login are the Current Release plus the two previous releases, all versions before these are now blocked!
One side effect to this is as a new version is released the older of the two previous releases will then become a blocked version.
Another side effect is, if you are trying to use a blocked version, it will update to the latest release before it allows you to login.
TPVs are being encouraged to adopt the automatic update / version management capability and / or to restrict their users to using the more recent releases of their viewer so as to lighten the load of having outdated versions of viewer which fail to leverage more recently improvements and capabilities (HTTP, interest list updates, fitted mesh, etc.). As such, UKanDo becomes the latest TPV to do so.