RelatedSiteAccessList and AvailableSiteAccessList are different setting, the first is a list of site accesses that are related to current site access, share db for instance, share content on multi side setup and so on. The latter is list of all available site accesses on the eZ Publish installation.
So they are not interchangable, the reason eZContentCache fallbacks to AvailableSiteAccessList is that RelatedSiteAccessList was added in 3.8 or 3.9 and fallback for existing setups where provided for bc (backward compatibility) at the time. Code written after that mostly use one or the other depending on use.
So eZContentCache should be cleaned up to only use RelatedSiteAccessList imho.