I would like it if my local government funded an alternative to Nextdoor, for instance.
I agree that the issue arises for current systems that allow broadcast (one-to-all) communications such as this forum. I also acknowledge that it exists for email (one-to-one), but I think only because in that case it's so cheap and easy to mass mail something that one-to-one has effectively been automated into one-to-almost-all. I'd also note that modern spam filters are quite good at their job.
But for example, SMS isn't meaningfully moderated in any way yet when was the last time you had cause to blacklist a number?
I don't see any reason why a newly designed system couldn't apply different techniques to (mostly) avoid the problem. Just off the top of my head there's personal whitelists and blacklists, federated systems (pick the centralized ruleset that works for you), community moderation (ex StackExchange), web of trust, the subscription blacklist model that adblockers use...
There is no way for me to stop the US postal office from delivering me giant catalogues of advertisements multiple times a week. I regularly get mail addressed to "current resident." I ought to be able to go to the postal office and say, "If someone doesn't know my name, just don't deliver the letter." It's friendlier for the environment, it would make the experience of checking my mail more pleasant.
The reason I can't do that is because spam is a giant source of revenue for the US Postal service. Coincidentally, the US Postal service has a number of services specifically designed to make it easier for advertisers to get mail into my mailbox. There is, in my mind, no reason to believe that a nationally owned social media platform would have any different incentives than Facebook would.
Email, a federated service that doesn't really make the government any money, doesn't have those incentives. So open protocols that are unaffiliated with any single organization (government or company) work better in my experience, unless whatever service is created is isolated from making profits in traditional ways. There are some good examples of that, but I'm not completely sure the Post Office is one of them.