Ah, so you’re looking for a way to quickly link/unlink all the cels on a layer? Yeah, the only way to do that is to select all the cels ): It might be possible to write a script to do it though.
(In the future, it would be easiest for others to understand if you state what you’re trying to do rather than just say the intermediate step you want. There are sometimes other approaches that don’t require the intermediate step.)
I think what would be nice is if you could use the Home and End keys (the same keys as the First Frame and Last Frame buttons) together with Shift to select everything from your currently selected layer to the First or Last frame. That would make it easier to modify large swaths of the animation, and would be more flexible for large animations than a button/menu to select everything. So, if you do want to select every cel in a layer, you could go to the first frame, select the cel you want, and then press Shift+End (or Shift+Last Frame button) to select everything from that cel to the last frame. If cels in multiple layers are selected, then all the cels between those and the respective final cels should be selected. It would work nicely with the current UI and and would not introduce new concepts.