[SINGLEPLAYER (didn't test in MP)]
Writing in regards of InSim ISP_PLL(23) packet not being sent sometimes.
It works in one case and not the other.

Case 1) It works when the player is still in the screen which appears after you press Singleplayer.
By clicking the "Join Race" button and then "Spectate" after that

Case 2) It does not work when the player is already driving and uses Shift+S or selects it via escape menu.

It is not clear to me if the player slot should be preserved when doing Shift+S while driving or not.
If the player slot is not meant to be reserved then it should send the packet.

Further testing proved that each time the player does Shift+S and Shift+J(Join), the player slot changes but ISP_PLL is not sent.

Or I have read it wrong and the concept of "player leave (spectate - loses slot)" is conditional

Edit: LFS Version 0.7F (Latest)

In this case the answer can also be “use a library”