https://gitlab.gnome.org/GNOME/gtk/-/issues/233
This is not about thumbnails in the file manager but about the file chooser (open/save dialog).
I believe the following post is still relevant:
https://jayfax.neocities.org/mediocrity/gnome-has-no-thumbna...
At least KDE does this properly:
https://www.ubuntubuzz.com/2021/04/comparison-of-file-choose...