Two issues related to the same bug.
First, the fact that the "lock" state of the Timeline window is undoable is inconsistent with the Inspector, Animation & Animator windows.
The second issue is that the Timeline is updated even if "lock" is enabled, when using Undo.

repo1:
1- Unlock timeline
2- Select gameobject with timeline 1
3- Select go with timeline 2
4- Lock timeline
5- Undo
bug: Timeline window is still locked, but timeline asset has changed (!)
expected: Undo of a GO selection should have no effect on the contents of the Timeline window since it is locked.

repro2:
1- Unlock timeline
2- Select gameobject with no timeline
3- Select go with timeline 2
4- lock timeline
5- Undo
bug: Timeline window is cleared and is no longer locked.
expected: Undo should not affect the lock state exactly like other tools (Inspector, Animation & Animator windows)