You can need to click again on your selection and the handles gets a different color.
Now you can resize without changing the aspect ratio

Hmmm... I did not know that (still have to read the docks). Maybe it would be valuable to add a lock icon?

I think it isn't mentioned anywhere in the docs. It would be nice if you could contribute it.
Where would you like to have the lock icon? I don't see where I could add it

Above the top right corner maybe? Or maybe next to the circle in the middle of the object?

Top right is a bit random. We already put the rotation circle at the top (see #607 in https://preview.butterfly.linwood.dev). How should this look like? Is there any reference to other apps?

Hmmm... I think it should appear inside the selected frame. It makes more sense to me.

The gimp uses a chain (complete/broken) and inkscape is using a lock symbol (closed/open) for (keeping/not keeping) the aspect ratio:

But there it isn't inside a canvas. I'm currently not sure if this looks good

It is not, that's true. But I still think that there should be some hint about the ability to lock the AR.