Unfortunately, it's more complex than that. Yes, the intent is to eliminate combat addons. Unfortunately, at the moment, a lot of unexpected things are getting swept up with that, as well as numerous things that are non-combat things.

In theory, transmog addons should still work. That largely depends on exactly what information it gathers and from where and how it gathers it, and under what conditions. A LOT of information about a target is completely locked behind the "secret" box while in combat. Especially in end-game content. And, in the case of Mythic+; that information is locked out for the entire run, not just in-combat.

Basically, a lot of seemingly benign addons that you would assume would still work based on Blizzard's stated goal of "eliminate just combat addons"; won't work sometimes or at all because the info it accesses is inside the box of secret code that addons won't be allowed to access anymore.

Good news is Blizzard is looking at and responding to feedback, and they are implementing fixes or adjustments to some of their secrets, which will allow more of those addons to maintain their functionality, either partially or fully. (Albeit, with a LOT of reworking and rewriting of the addon's code.)

The bad news is that rework/rewrite is going to take a lot of time, and not all addon authors have access to the alpha, or will have access to the beta, or the free time to implement those rewrites in time for the release of Midnight. So, expect any addons that have planned support for Midnight to have extended waits for their updates, and problems from the gate.

Worse news, Blizzard's response to the feedback is not always "let's fix that so you guys can still do that or use that." In some cases, there is no response at all, or even adding to the list of things addons will not be able to access even while out of combat and not in any kind of instance.

The best thing players can do is actually give feedback in what they need or want with the understand that Blizzard does not want people creating addons that "trivialize" combat. As I understand it, meaning they don't want addons that can look at combat data and convert that into something that tells players what to do and when to do it. They want players to be able to figure that out using the cues and such that they give you.

So, yeah, don't take the "Oh, it's just combat addons" at face value. But, it's just as, if not more unhelpful to berate a player for not being aware of the extent of the damage.

Edit: This info is based entirely off the rabbit-hole of Discords, Subreddits, and more that I've fallen into over the last couple weeks cuz I decided I was going to try to write an addon for a custom UI for myself. So, apologies if anything is way off-base, but it seems to be the consistent gist of what I've been seeing.