The person who heavily uses workspaces and always wants to show workspace thumbnails even there is only one workspace after upgrading to GNOME 40.
Since GNOME Shell 40, they (upstream) decide to hide workspace thumbnails when there is only one workspace, because "there are people who don't use multi-workspace at all so we need to support single workspace case better".
I personally think this is a bad idea, because multi-workspace is a feature and when people don't know a feature, we should tell them about it instead of hide it.
I also think that showing workspace thumbnails has little influence to people who don't use multi-workspace, but may make people who use workspaces heavily unhappy, so I create this simple extension to always show them.
This extension just hooked ThumbnailsBox.prototype._updateShouldShow and always set true when called.
Install this extension, and enable it in GNOME Extensions app.
I have another extension and you can refer to its README.md for the answer.
Upstream already made a design for the new Shell, so it's hard to argue with them and I prefer to do before I say.
Send an issue, I will upgrade it if I am still using it.