I did find an 'installdate' in the softwares table. Where it is set, the latest date something was installed at least establishes whether any maintenance has been done at all. However many systems are leaving that field as zeros - probably due to running older agents or operating system differences.
What I'd like ot see for history tracking is a way to make a text representation of the whole invenory of a node minus the parts that change routinely like the inventory timestamps, disk space, etc. and pass it to a version control system like git or subversion. Ideally you could pick a system of each basic type as the starting point and add similar systems as branches so standard version control operations would be able to easily show differences between any two systems as well as in a single system over time. The challange there is to make a text where the diffs woulld be humanly readable. XML diffs don't make much sense, and table representations don't work well for elements with variable numbers of values.