To what exactly they are tied?
I could not play for a while so as I logged in today at my base it was decayed but at the 24h grace period, luckily no one teared it down so all is well. According to the log all thralls at the base decayed immediately at the time the grace period for the buildings started (“owner offline for to long“), which is the expected behaviour as far as I know.
But I have a small outpost two map squares away which should have decayed even earlier because I logged out at the main base. As I reached the outpost the grace period was active as well but with some hours more left, and the thralls at this place (barkeeper, cook, some guard) were still there.
And there are some thralls in the wild I rescued from cages and parked for later retrieval, these were also alive and well.
So maybe thrall decay is tied only to the base were your bed is? This would also explain all these abandoned poor souls floating in midair long after bases are gone…
(All on official PvE)