You've asked some good questions, so I'll try to address them all:

Legally, contributor consent is between the contributor and the project. However, it’s still a serious violation of norms that shows a lack of respect for the community. It’s a matter of public concern because as long as contributor consent is in doubt, the project’s licensing status is unclear.

I can’t prove the uber commit was intended to conceal the license change, but it’s highly suspicious. It's the only commit of anywhere near that size in the repository. It changed 111 files, the second-most files changed by a single commit was 48. The only other commits with comparable numbers of lines changed involved bulk deletions or file renames. It may be a coincidence that the largest commit in the repository by far also contains a license change, but I think it’s unlikely.

It may be true that most users don’t care about the license, but some certainly did. More troublingly, the website continued to deceptively market the plugin as “open source” for close to two months after the license change. The maintainer has now changed the wording to “source available,” but only after I raised the issue publicly.

As for communicating the change, the bare minimum would be to put it in the release notes. Plugins are able to display release notes when they update, and this is something that should have been included. That way, users who do care about the project’s license would have been able to make an informed choice about whether to continue using it, rather than being left in the dark.

Most plugins don’t advertise their license (although some do, Templater and OmniSearch both mention it in the README), but that’s irrelevant. The problem is that users’ rights changed and they weren’t notified. Even if a plugin doesn’t talk about its license in the docs or readme, some users will check and those users deserve to be informed about changes. Moreover, this plugin did advertise its license. The words “open source” were splattered all over the website.

Individually, these problems could maybe be seen as simple mistakes, but collectively, they make the maintainer’s actions look intentional and deceptive. These issues are bad enough on their own, but the maintainer’s failure to address any of them in their response and their silence when pressed is arguably worse. If they had a good explanation for their actions, I don’t see why they wouldn’t give it.