Inactive users and communities will be notified that unless they take action (logging in, posting entries) they will be deleted & purged -- however, "inactive" is very narrowly defined in the actual plan.
The account must be (if a personal journal) not logged into, for 24 consecutive months; a community must go without new entries for 24 consecutive months. Users will be notified before this happens; community maintainers and moderators will be notified. Additionally, only journals with either zero entries or the single automatic entry that LiveJournal started posting to new accounts, will be affected. If a journal has more than one entry in it, it's safe. If a journal has only one entry in it, and it's something other than the automatic text supplied by LJ, it's safe. The feature isn't fully developed yet. Stay tuned to official LJ sources (which I am not).
Historically important stuff is therefore likely to be safe, unless somehow a world-shattering conversation broke out in the comments of someone's automatic first post (and those tended to actually be automatically posted privately, so immensely unlikely).
This also means that the people who were pleased by the thought that the person with three entries from 2001 who has that awesome username but hasn't touched their journal since 2001, will actually be in for a disappointment.