I agree it is difficult in the situation you describe, which is not mastodon.technology's situation.
I'd created both a subreddit (https://old.reddit.com/r/plexodus) and wiki (https://social.antefriguserat.de/index.php/Main_Page) to memorialise at least some of that discussion, as well as the on-platform discussion.
That last is of course now unobtainable, though there were over 700 snapshots of the community homepage saved to the Internet Archive's Wayback Machine, see:
<https://web.archive.org/web/20190401071102/https://plus.goog...>
Because of the mechanics of both G+ and IAWBM, actually navigating to articles from that archive ... mostly does not work.
The G+ community had over 4,000 members (pretty good for late-stage G+). The subreddit ... a hair over 300.
People. Will. Not. Go. Off. Platform. Even. If. That. Completely. Fucks. Them. Over.
It's a really perverse dynamic.
At the same time, based on my own personal experience, it is absolutely impossible to get people to move to another platform or service even for the purpose of discussing future plans.
My exceedingly strong advice is to have multiple points of presence defined as a matter of course, one of which should be a simple email list (which provides persistent contact information), regardless of any awareness of an impending platform shutdown.