In terms of recording the state of the game it's all the same: a group of variables used to store the values of individual elements/quests/plot sub-paths etc. It's not like porn sandbox games are inherently unable to use older saves, plenty of them do it without any issues.
The incompatibility only enters the picture if for some reason developer decides to mess with already established variables and/or structures and how their values correspond with the game state. Avoiding it is just something that should be taken into account in advance, when the game framework is planned. And if this sort of mess happens with every subsequent release, then it's something wrong with the project on fundamental level, even more so if it goes unaddressed and the player is expected to just deal with it.
I confess that i have trouble envisioning how i'd go about designing a game that'd break the saves even on any slightest change or addition. Sandbox or not. In a way, it's almost impressive that this one manages such feat. Well, unless it's just some simple case of checking version and if it's different then rejecting the save, no matter if it's actually incompatible or not. But that would be silly.