To address the question in the title:
Is EM theory appropriate on a physics forum
Yes, it is. The electromagnetism tag on this site has 12 thousand open questions. This is the second biggest tag on the site (with QM having 20k open questions). Any claims of "discrimination" or "bigotry" against electromagnetism are entirely divorced from the reality of the site.
That said, just because a question or an answer involves electromagnetism does not automatically imply that it is on-topic $-$ it also needs to be in line with the other site guidelines. This much should be obvious: like all venues, there are some ground rules, and posts that don't satisfy them get closed.
In particular, when you say
Is this in line with SE's charter, to be a good community forum
you point at what is likely the biggest disconnect here. The basic charter of Stack Exchange sites is not to be a good community forum, or indeed a forum at all. This is a questions-and-answers site. The goal is to be a place where people can ask questions about physics and get high-quality answers, and where future visitors can learn from those answers.
In particular, this implies that there are several other things that this site could be that have been rejected in favour of the core goal of being a high-quality, high-signal-to-noise Q&A site. Among these:
Questions about history are considered off-topic because there is already a dedicated site for them. Questions here should be about physics and not about its history. This keeps the site focused.
Homework and exercise questions are considered off-topic, because they significantly reduce the signal-to-noise ratio in the quality of questions, which reduces the engagement from expert users.
This site is not an appropriate location for peer review, because we cannot do it in an effective way and it is a very poor fit for the format.
More generally, discussion threads are off-topic, because they are a poor fit for the format that comes from being a Stack Exchange Q&A platform. If your goal is "I would like to know X", then the platform is well suited to finding, sorting and rewarding answers to that question. If your goal is "I would like to participate in a discussion about X", that doesn't really work with the SE software: it produces endless threads which are hard to navigate, and with endless activity that crowds out more pointed questions. We're not saying that discussion is bad, just that it doesn't fit here.
Non-mainstream physics is off-topic here. The site scope is physics, as it is understood in the mainstream literature. This is because if non-mainstream physics is allowed (say, if questions and answers taking flat-Earth theory as correct were permitted) this site would lose all of the experts in its community in short order. This is a hard line to draw in an objective way, so sometimes some toes end up getting stubbed in the implementation, but it is an essential component to keeping this site viable, as I'm sure you can appreciate.
In addition to that, questions here are expected to be readable, useful, and clear, to suitably back up any assertions they present, and to contain actual questions. Low-quality posts that fail to meet those standards sufficiently get closed, in service of keeping a high signal-to-noise ratio for potential answerers.
(Obviously this is a non-exclusive list.)
Those are the ground rules that this site has used to become a quality resource and a place where one can get answers from extremely valuable experts in a huge range of topics, starting with electromagnetism and quantum mechanics. If those ground rules suit you, then we'll be glad to have you! If you're looking for something different (say, a venue that can accommodate discussions or peer review), then we hope you find a venue that does suit your needs.