Now, my default file type for Visual Studio Code is Plain Text.
If I save a new file with a name like a.in, it will save as a.in.txt, and I have to change the dropdown to All Files.
I found the same problems in github: 1, 2. However, they seem not to solve my problem.
This is not possible in the general case in VSCode.
The issue is that you cannot assign "no extension" to a language, and as per the links you mentioned, the All Files (*.*) option is disabled by upstream (electron).
Therefore, you will either have to remove the extension manually, OR you can create the file first (using the terminal, Explorer, an extension, etc.) and then open that existing file.