Thanks for reaching here! If you have enabled Automatic scaling and seeing a lot of 404s to /admin/host/ping in your app service then its expected behavior as this feature is still in Preview.
To add to this further-
It will not affect scale-out. Only the status code 429 will impact scale-out. Otherwise, scale-out is determined by latency.
This issue will be fixed in a future release, which will reduce the noise.
In the meantime, the 404s should not have any negative effect on scaling, except for the noise. These pings are also not a security risk for the app, as they come from the same scale unit that hosts the app.
Hope this helps.
Let us know if further query or issue remains.