Not enough details. Why exactly are those DOM tree elements not garbage collected? Something must be holding a reference, I wouldn't expect just there being <a> tags to be the cause for what you see.

routes.cljs: 
------------ 

(defmulti panels identity)
(defmethod panels :default [] [:div "No panel found for this route."])

(def routes
  (atom
   ["/"
    {"/posts" :posts
     "/user" {["/" :id] :user}}]))

(defn dispatch
  [route]
  (let [panel (keyword (str (name (:handler route)) "-panel"))]
    (re-frame/dispatch [::events/set-active-panel panel])))

(defonce history
  (pushy/pushy dispatch parse))

(defn navigate!
  [handler]
  (pushy/set-token! history (url-for handler)))     



views.cljs:
-----------

(defmethod routes/panels :posts-panel [] [posts-views/posts-panel])
(defmethod routes/panels :user-panel [] [posts-views/user-panel])

(defn main-panel []
  (let [active-panel (re-frame/subscribe [::subs/active-panel])]
    (routes/panels @active-panel)))


posts/views.cljs
----------------

(defn posts-list []
  [:<>
   (let [data @(re-frame/subscribe [::posts-subs/data])] 
     (if (empty? data)
       [:div "No posts here"]
       (for [[index d] (map-indexed vector data)]
            [post d index]))])


(defn posts-panel []
  (let [query-params (re-frame/subscribe [::subs/query-params])]
    (re-frame/dispatch [::posts-events/http-get-post-list @query-params])
    [:<>
        [posts-list]]))

Hold on. Previously, you mentioned "DOM elements not being garbage collected". Now you tell that those elements do actually exists in the DOM itself and are visible. "Not garbage collected" in this case means only that they're present in the memory. The fact that they are still rendered is the exact reason for why they're not garbage collected, so you should focus on why the elements are still rendered. Unfortunately, your code is still not nearly enough. But seems like finding the issue will require quite a bit of code so what I can suggest is to spend some time and create a so-called minimal reproducible example. One that contains everything that's needed to reproduce the issue, but not more - it's much harder to dig through a full-on app rather than through an isolated reproduction case. Once you do that, share a public Git repo and I'll try to reproduce it.

I think I managed to get rid off this issue. I stripped it down to minimally reproducible example, like you said, and and then worked my way back. In the end it didn't have anything to do with Reagent nor Cljs. It came down to mostly two things: conflicts with legacy javascript (which I had to include into the page along with my Cljs app) and React version. Once I upgraded to React 18, along with getting rid of some components from legacy code, the number of detached elements became reasonable. What I mean by reasonable is that it didn't double every time I switched between two views components back and forth. They weren't rendered on the page anymore but they weren't being garbage collected either so their numbers were growing more and more.