I poked fun at the Bruins in this thread, but honestly I don't see it and I think its likely garbage. The thread is 2 and a half months old, and if the Bs were actually thinking about signing him (to a contract) it probably would've happened by now. And while I don't know much about Haggerty, I've seen enough others say that he isn't all that reliable to think there is something "real" here.
That said, I do see the idea of a PTO not being absolutely terrible going into this season, just to see if he has any NHL skill left at all. The unpredictability with COVID and the ability to have the taxi squad would have it make SOME sense to at least have an experienced veteran on easy standby. I'm guessing Alzner won't be all that picky about a reduced role. Being realistic, the Bruins have some good young talent on D, but not really a lot of experienced depth. If they're intending on competing this year, having to rely on multiple rookies at the same time for any extended duration may be pretty painful.
(That second paragraph could honestly apply to a number of teams. Rebuilding ones that just want a veteran around could do worse. I've read that, along with his body being pretty beat up, Alzner found it very hard to adapt to the zone defense system Claude Julien deployed in Montreal, having come from man on man in Washington. Perhaps he could not suck horribly for the right team.)