ntppool.org has a “News” section. Once an announcement is published, a link to that page can be shared to news sites, Slashdot, Reddit, what have you nowadays. There’s also a mailing list (although it may have issues at the moment but the subscriber list is probably still intact if the server comes back online some day) and the comp.protocols.time.ntp newsgroup. It may be true that we may never reach all the end users, but I’d think we’d be able to reach a significant part of device manufacturers and ntp software authors, who could then figure out if the change affects their devices/software and inform their own end users accordingly.
I don’t think disseminating the news is the biggest problem here. There would need to be a decision about the planned dates first. What I posted above was only my suggestion.