No, but I am unsure that this qualifies as doxxing.
Although this answer argues that posting the name of the purported whistleblower is not acceptable or advisable, I'm going to argue differently. I think the Acceptable Use policy leads to precisely the opposite conclusion, and that even from a broader perspective there's little reason to omit the information.
First, let's look at the section of the Acceptable Use policy mentioned previously:
Identity Theft and Privacy. Users that misleadingly appropriate the
identity of another person are not permitted. Users may not post other
people's personally identifying or confidential information, including
but not limited to credit card numbers, Social Security Numbers, and
driver's and other license numbers. You may not post information such
as other people's passwords, usernames, phone numbers, addresses and
e-mail addresses unless already publicly accessible on the Web.
I think we can discard the notion that "personally identifying" information is meant to cover publicly available names. For instance, if I say that Mark Felt was probably Deep Throat, or that Individual One is likely Donald Trump, that is not a violation of the policy since these individuals' names are widely known. In other words, the confidentiality portion is important.
However, contrary to what the other answer argues, the name of the whistleblower has not been "protected with the highest levels of confidentiality (despite attempts by others to name them)." On the contrary, their name has been 1) tweeted by the president of the United States, 2) tweeted by the US Senator, 3) bandied about on live TV, and is 4) currently visible on, by my count, at least 160 webpages (The search results said over 100,000, but when I went through there were far fewer. Perhaps this reflects aggregation of domains with a huge number of mentions).
The "confidentiality" mentioned here mainly consists of various major news outlets declining to use the name in their stories, due primarily to their own specific journalistic codes of ethics. The majority of people who don't know the name of the purported whistleblower don't know because they have chosen not to go to the mainly dodgy far-right sites where the name is mentioned extremely regularly.
And this brings us to the second half of the acceptable use policy, which implies that if otherwise personal information is "publicly accessible on the web", it is acceptable to post, even passwords and usernames! The unavoidable fact is that, with over 100 websites giving the name of the alleged whistleblower, it is very much publicly accessible on the web, and is only likely to become more accessible. Although the sites that mention this person's name are, to my estimation, primarily far-right or conspiratorial sites, this is not a universal characterization: the Dallas Morning News, with a somewhat conservative but highly anti-Trump editorial board, also has an article that mentions their name, for instance. This information is also now available on CSPAN and in the Congressional Record. It is emphatically no longer private in any colloquial sense.
In short, based on the demonstrable fact that the name of the alleged whistleblower is publicly accessible on the web and has been mentioned by politicians on Twitter and in person to literally hundreds of thousands or millions of people, I think this does not fall into the category of confidential information as classified by the Acceptable Use policy.
We also need to consider the portion about defamation. That portion says:
Hate Content, Defamation, and Libel. Hate speech and other
objectionable content that is unlawful, defamatory, and fraudulent.
Note that an allegation of defamatory expression, in and of itself,
does not establish defamation. The truth or falsehood of a bit of
expression is a key element in establishing defamation, and we are not
in a position to make that sort of fact-based judgment. That said, if
we have reason to believe that a particular statement is defamatory (a
court order, for example), we will remove that statement.
It's clear from this merely alleging that a statement (say, Rand Paul's tweet about the purported whistleblower) is defamatory is not sufficient. Even if I feel it is, or even if the whistleblower themself does, the policy emphasizes that this, by itself, is not enough to prevent any mention of the statement, in the absence of a court order. Further, in line with the US legal notion that truth is an absolute defense to an accusation of defamation, the Acceptable Use policy requires that the statement be fraudulent. Thus, in what I consider the quite probable case that the purported whistleblower is in fact the actual whistleblower, this would not constitute defamation under SE's policy. Moreover, even if a tweet or article by another person could be defamatory, merely noting their allegation would not necessarily be defamatory under US law, which is most likely to apply to SE posts.
In conclusion, I doubt the defamation portion of the Acceptable Use policy requires its removal.
Of course, the acceptable use policy is only one aspect. We should also consider whether it's ethical to post the information. On the one hand, we need to consider the possibility that mentioning the name of the purported whistleblower could lead to a risk to their safety. I consider this implausible. First, the government officials who are clearly motivated to retaliate against them are clearly well aware of who they are. Second, the much larger but much less motivated group of potentially dangerous far-right followers of Trump are also well aware of their identity, because their version of newspapers like the New York Times are conspiracy sites like The Blaze, which have had few scruples about sharing the name of the alleged whistleblower.
On the other hand, we have the fact that a person who has divulged information that led to the impeachment is inherently newsworthy. See the aforementioned Deep Throat. In addition, there's a possibility of a Streisand Effect, whereby the paucity of information about a fact increases its popularity: even, in this case, potentially driving people to highly unreliable news sources due to the relative absence of the information from trustworthy ones.
To summarize, I think mentioning their name contributes little if at all to informing the people who would threaten them, and promotes the cause of providing the public with trustworthy information about newsworthy events. It's also quite unlikely that this constitutes defamation according to the SE policy. In conjunction with the idea that the Acceptable Use policy does not prohibit divulging their name, I think it is acceptable to include in an answer.