Describe the project you are working on: A 2D rhythm platformer
Describe the problem or limitation you are having in your project: The GDScript Style Guide page on the documentation says that per the style guide, GDScript file names should be in snake case. However, the 'Attach Node Script' dialog suggests the same name as the node, in pascal case by default.

Describe the feature / enhancement and how it helps to overcome the problem or limitation: To achieve consistency with the documentation, the autocompleted file name should be converted to snake case.

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams: From what I have heard, Godot already has case conversion methods, so it should be easy to just convert pascal to snake case.
If this enhancement will not be used often, can it be worked around with a few lines of script?: N/A
Is there a reason why this should be core and not an add-on in the asset library?: It is more consistent with the (core) documentation.
From what I have heard, Godot already has case conversion methods, so it should be easy to just convert pascal to snake case.
Those methods exist, but they are not part of core currently, see my proposal #719.
I think scene names should be snake_case too, otherwise it doesn't make much sense. And the script name already uses scene name, so they are consistent.
To anyone implementing this: The convention should be changed automatically depending on the target language. It should not apply when using C#, which uses PascalCase for file names (the file name should be the same as the class name).
pls no this should be a option not a requirement
pls no this should be a option not a requirement
@Shadowblitz16 Both would still be an option. This is just suggesting that the default option should be consistent with the documentation.
I think scene names should be snake_case too, otherwise it doesn't make much sense. And the script name already uses scene name, so they are consistent.
@KoBeWi I might be wrong, but doesn't it use the name of the node you attach it to? (which should be PascalCase)
Also, I previously had been using PascalCase for scene names, but I think you're right. If Godot as an engine wants to start formally autosuggesting snake case for script names, it should do it for scene names too. The snake case next to the node icons in the editor itself will take some getting used to, but I guess it's consistent.

EDIT: Apparently the script does suggest the scene name, but only if it is the root node. Otherwise, it suggests the name of the node. This means that if you use snake case for scene files, the root node will have a script name suggested in snake case, but all other nodes will have a script name suggested in pascal case.
I might be wrong, but doesn't it use the name of the node you attach it to? (which should be PascalCase)
Actually it's both. It uses scene name when script is attached to root and node name for other nodes.
That's funny. I just realized that and edited my comment, but before I saw your reply just now.
Most helpful comment
To anyone implementing this: The convention should be changed automatically depending on the target language. It should not apply when using C#, which uses PascalCase for file names (the file name should be the same as the class name).