My understanding is that, possibly in response to this ping, Tom Breloff wrote a blog post which reaffirms the information that he is not actively involved in Plots.jl at the moment. So pinging made him spend time (which, apparently, is a scarce resource for him ATM) on confirming something which was already known, or could have been found out with relatively little effort. I can imagine that the blog post is a polite way of letting people know about the situation, so that he is not contacted about it repeatedly.
I am not sure this was helpful to him. Sure, this is not the end of the world, and getting yet another e-mail can be dealt with easily. But these things pile up. My concern was (and is) the following: if, instead of using this forum and Github issues, unwarranted direct contact of maintaners (or ex-maintainers) of packages becomes a norm in this community, it could be a bad experience for package maintainers.
One of the implicit understandings of (unsponsored) open source software is that the time of contributors is a gift. We can make no demands on their time (however minor), and must make every effort to streamline the administrivia they have to devote to the software. This also benefits the community, because they can spend their time improving the software. Pings and PMs result in e-mails, which are the worst possible way of communication about software. That’s why we should use them sparingly.