I generally think these are a positive thing, because they clear out space and make usernames available to people who want them. They also mean, however, that you should not only log into old accounts and comms you want to preserve but post a message deliberately saying "LJ is cleaning house, this is to prevent this journal from being removed".
LJ has designated that "A journal is defined as inactive if it has not been logged into for 24 consecutive months and has only one post. A community is defined as inactive if has not been updated for 24 consecutive months and has only one entry and no comments." Thing is, I don't actually trust LJ as far as I can throw them where policy and definition is concerned (the above quote is already a correction to earlier policy) so I feel it's better to log in and make a brief post even if the journal in question does not fit the description. It's best to treat LJ as a capricious five-year-old: given to mood swings, lacking impulse control, and impervious to subtlety.
So, steps I took: I have a couple of old accounts which, while not currently active, are part of roleplays and should stay in that record; I also have comms such as gta_of_doom, which is inactive now that I'm no longer a GTA but which I would be sad to see go. I logged in, posted a brief message with comments turned off, and logged back out. I also got a lovely dose of nostalgia for some of my old shenanigans.
Also, if you are the controller or know people who control journals of those who have passed on, you should take steps to make sure their journals are preserved (if you wish to have them preserved, anyway). They can be tagged as memorial journals, apparently by contacting firstname.lastname@example.org; please bear in mind that the last time I contacted support, my request was immediately posted to the public support area of LJ, so proceed with caution.