Google does index this forum, though its coverage of the mailing lists has dramatically declined over the years, so I’d expect its coverage of the forum to be similarly scattershot.
Anyhow, this was a rhetorical device; the point is that many people, especially those on the periphery of this project, feel more comfortable in an environment sheltered from the prying eyes of social media and search engines. The culture of civility and empathy there exemplifies the best in OSM. But because of the login wall and fast-paced action, it’s better-suited to sharing advice, links, and funny photos than long-term technical decisionmaking.
If someone did get a big 
from the community on Slack, they should be able to produce a link to the discussion. Best if they can provide that URL upfront in their changeset tags. Five DWG members are members of the workspace and can ensure that the mapper didn’t just ask ChatGPT to fabricate a Slack URL for them and that they didn’t post in #all-alone without getting a response. I recall an episode a year or two ago when someone on the talk-us list tried to pass off Slack as having already acquiesced to their import idea; they were quickly called out for it.
At the end of the day, someone making a major edit is responsible for doing due diligence and hearing out a range of perspectives in whichever venue can facilitate that dialogue. On a practical level, they would need to somehow involve the OSMUS Slack workspace, even if it’s just cross-posting a link to a forum or wiki discussion, because Slack dwarfs any of the other relevant communication channels (edited with stats from the summer doldrums):
But I want to emphasize that, even though the atmosphere is very different on Slack than the mailing lists or forums have historically been known for, we’re all part of the same project and our shared goals overlap much more than we give ourselves credit for. The sky didn’t fall when we recently started allowing import discussions to take place on the forum instead of the mailing lists. Nor did it fall several years earlier, when the majority of active talk-us participants invited themselves into what had been the OSMUS board’s private Slack workspace for day-to-day work, simply because it met the pent-up demand for real-time chat.
This forum did exist in some form back when the community reached a consensus with their feet that Slack was the better alternative. Times are changing, but I think we can work towards a more open recordkeeping practice without rushing to delegitimize the consensuses that we’ve achieved on purely procedural grounds.