Used LuauHeap -> Unique References to finally lock in a client side memory leak where every player added an untracked player instance when they left. Turns out it's with Roblox core scripts: CameraModule, and Popper.

How to use the Developer Console to look for LuauHeap Memory Leaks and finally unlock your game's potential: 1) Open Dev Console and tab over to LuauHeap 2) Decide if you're looking for client or server leaks. 3) Click Create Snapshot 4) Tab over to "unique References". Roblox will show you what garbage collection can't clear. 5) Now you know what memory leaks you need to fix! *Note that with so many edge cases as the cause of memory leaks in general, and as lines change in your test server vs main server, publishing memory leak fixes over multiple updates to gather multiple batches to fix makes this less daunting.