The General Answer To Your Question
To give feedback on mod behavior, you would cite specifics in a meta question or an answer to a meta question about mod behavior, citing the behavior and why you think it's against the rules or otherwise undesirable. Not threadcrapping in a meta about something else, not on the mainsite in comments, not in comments on a closed question, or anywhere else that is not the right place for unrelated content in general. (To use a gaming analogy, you know how people say "don't argue rules with the GM at the table, bring it up after?" There's a right time and place to contest actions and "in whatever comment thread is at hand" isn't always it.)
You would also do so by addressing the behavior and not attacking any individuals or groups. Incidentally, this is how you'd address questionable behavior from any individual or group on the site you believe to be misusing the system or violating Be Nice. (And of course to officially contest something like a suspension you can also escalate to the CMs).
Your Specific Case
While in general we are not at liberty to share details of user suspensions and similar, a recent meta.SE has said that we can share some minimal details when a suspension is questioned in the interests of transparency, and since the user in question has brought it up, I would like to assure people that such actions are not taken quickly or lightly.
In this case, a first suspension was issued by a mod (in consultation with the rest of the mod team) after a series of issues (both on main and meta). Several mod-messages had already been issued explaining the issues with site behavior, one as recently as April 7, 3 days before it came up again. Your last comment (in the comments on a closed question) was part of a continued pattern of inappropriate commenting (both in tone and in nature - "extensive discussion in comments" and "be nice" are both relevant). When a problem is recurring, the "last straw" isn't always the most egregious. When you inquired about your suspension via mod-message all these multiple issues were linked to you again and explained.
We don't expect you to always agree with the mods or to be happy about your suspension, that would not be a reasonable ask from us. But if you simply constrain your issues with us (and others in the community) to the right place and address issues instead of attacking people, it'll all be fine. I'll note you've done exactly that with this question!
About Be Nice
A couple important notes to the community about how Be Nice is enforced, just in general (these quotes are hypothetical and not from nvoigt's content, though are derived from real comments etc. we've seen on the site).
Attacking groups isn't better than attacking individuals. "Oh this community here is so stupid" or whatever is Not Nice. This was brought up in the Q above, so it's worth saying that a specific attack directed at a person is certainly bad but generalizing it to more people doesn't make it better. It also applies to people and groups not on the site. "All the mods on ENWorld are morons" isn't OK either, even if you somehow know for a fact none of them ever read the site (hint: you don't).
Attacking mods isn't better than attacking other community members. When writing, consider - if you saw a comment where someone called some other site member "a bigot" and told them "they'll never learn" etc. you'd flag it and expect that person to be told to Be Nice. We're not just taking a mulligan if that comment is about a mod. If we're all supposed to be peers here, and mods are also to Be Nice, then you are also to Be Nice to mods.
Small attacks over time count. They get responded to, deleted, annotated, you get mod messages - please don't take these prior more gentle corrective steps to mean "if you suddenly get even worse and more offensive we'll do something." They mean "if you keep doing what you've been doing we'll do something." The goal of these steps is to show how the behavior is affecting the community so that you can alter it; doing the same right after a mod-message to that effect demonstrates that the lesson isn't being learned.
Be Nice also counts on meta. Or at least, "be respectful," to quote the SE meta about discussing suspensions.
Correct misinformation, but don't go out of your way to make people look bad. Focus on the evidence of what happened and avoid assigning motives. Assume good faith and take the time needed to remain civil.
It's not hard to discuss behavior instead of people and say you think something's wrong without hyperbole and attacks. If you'd expect trouble from calling someone something on the main site, you should expect the same trouble from doing so on meta. There are plenty of meta questions about "Why was this question closed" &c that can be used as examples. (Be Nice counts in chat, it counts in replies to mod messages... There's not a place on SE that it is suspended.)
We Get It
We understand that people feel things deeply about certain aspects of this site and gaming and whatnot, and that it's easy to get heated up when discussing something you're passionate about. We know it can be hard to stay objective, which is why there's a good length runway of steps before we get to suspension. And even suspensions aren't meant to be a "punishment" - when issued, they say "to cool off." Believe it or not, a good number of times we have to issue one, we get a message back saying "OK, you're right, sorry, I was getting worked up." Sometimes it's OK to need to cool down. You're not going to miss anything - all the new Q&As issued over that time are still here, etc.
Just please understand that everyone - not just the mods and the regulars on meta and those into the "inside baseball" of the site, but the casual users, the Googlers, etc. - are here for gaming content for their hobby, not arguments and drama and such spread all over the site. Instead of a comment on a main question about "this site sucks so bad and the community is all dumb..." - even if that's what you're feeling - take a couple minutes off and then if there's an issue bothering you enough you feel like it should be addressed, post it here on meta just like @nvoigt has done with this question - this is great and the right way to question something.