a high-rep user ... complained about being notified as "@JohnDoe" claiming (or at least implying) this was impolite and they should be notified as "@John Doe".
That remark was part of an inter-user war and as such had only a tangential connection to spaces in @ notices. As a comment on spacing it is almost meaningless (even if correct from some points of view), but as an indicator of the state of meta.MSE and the direction the whole site has been moving, it is worth a closer examination.
The remark about names and spaces was there as part of a longer stream of comments, chained through many questions on the main and meta sites, that aggressively deconstruct and criticize every single recent action of another user (the OP of the question where the remark appeared). Criticism was based on new invented standards fabricated for the concerted targeting of that OP by several dedicated "followers" from the meta. Examples of the new standards: this previously undisclosed rudeness of @FirstLast pings without spaces; a quota (applicable only to the targeted OP) of one answer per question, including answers in comments; new types of "context and effort" requirements not applied to others; and a novel requirement (naturally, used only on that OP) to specifically anticipate, list, and exclude every form of answer that OP might know about, or that others think he should have known about.
Given my understanding of the system (everything after a space is ignored) this strikes me as somewhat unusual and I never came across anything like this so far.
There is a reason you didn't see it before: it is a triviality that becomes a subject of criticism only when it can be connected to the one targeted user. The author of the complaint never made a point of it before although he must have received a large number of @ notices with the auto-inserted username.
While something like the above was pointed out to the user they did not acknowledge it in any way and moreover the comment complaining about the usage received some upvotes.
The upvotes are from two sources, one general and one particular.
In general, there is the meta.MSE-driven "Vote Trolling" phenomenon where any negative comment directed at a user who has made himself unpopular often enough on the meta, is very likely to get several free +1's, independent of accuracy or honesty of the remarks. In fact, the less honest and more aggressive, the more the votes tend to accumulate.
In addition, the targeted OP has a pack of 3-4 aggressive "followers" who have been on an individual and collective mission to close his posts on the main site, close and delete posts on the meta site, and contest everything in very long chains of comments. The followers have the warm and reliable vote support of several others, so that there is almost a guaranteed 5 or more upvotes on any negative comment toward that OP, and a similar (though slightly smaller in total vote count) trolling against the other unpopular folks.
Of course the problem with the political users running amok is that they tend to drive out the more apolitical types, and over time there is a phase transition to a kindergarten of vote wars and comment trolling. The meta has more or less reached that point and it will be interesting to see whether it can make its way back to normalcy.