We already have placeholder avatars. They're the initial character of the username, on some algorithmically chosen color.
The problem I see is that those placeholders aren't appearing for you or for me, even though they do seem to appear for Slartibart.
The main difference in reported device info so far is Slartibart is running 17.6 beta iPadOS, while you and I are on earlier versions. I'll also add that my iPad is a mini 5, which is different from the other two models so far. My browser is Safari.
I'll also note that Slartibart is using the MR Dark theme, while we're both using the Light theme. I temporarily switched to dark, and the problem still appears there, so it's not related to that (probably).
To further clarify where I see it, I started at this page:
I then scrolled down thru New Posts until I saw a thread with no avatar. Here's a screenshot:
In this list, I think the avatar of the most recent poster is shown next to the thrad link. I don't actually see anything consistent, so that's just a guess. The name of the thread's OP appears next to the date, which is the thread start date.
The "RCS supported carriers" thread shows no avatar. The thread OP has no custom avatar. The same goes for the "I never thought" thread. Curiously, the OP of the "Nitro" thread also has no custom avatar, yet an avatar appears, but it's not for anyone I see in the thread.
(The "Edit" checkbox is a mod-only control, so we can do things like edit thread titles, stickiness, etc. Non-mods don't have this.)
(Edited above to clarify apparently random avatar appearance.)
If the problem is solved in the latest Safari (17.6 beta), then that should eventually reach everyone, so adding a feature to the forum software would be superfluous.
If the problem continues to appear in Safari on 17.6 release, then someone on staff will send a bug report to XenForo (forum software vendor).