I suppose someone could maintain a series of "stable" repositories that are (at time of creation) essentially copies of the main void repositories. A simple shell script could be used switch releases on the users system. Security updates may then simply be backported into previous releases.

However, to ensure that updates between the releases do not break anything would be a HUGE commitment, very time-consuming and with limited resources (time, manpower) there is still always the possibility that some problems will be overlooked. An additional mechanism would probably need to be implemented to (semi)automatically deal with any update conflicts that cannot be avoided in the packages.

At that point you would need to update step by step, one release at a time, even if your systems current state is several releases back. Otherwise every new release has to be tested/fixed for all supported prior releases, which would multiply the effort to do so.