Back in the LAA/SABRE-days, whenever a flight was not full and a pax had the INF-edit code; the system automatically set an "I"-block for the adjacent seat, in the seat map.
Not sure, if this is still the case nowadays...
ok, played around a little bit. A few weeks ago, I had added my daughter as a lap infant for two itineraries, and noticed that the middle seat was still available to select after I had done that. Then it dawned on me this morning to try changing seats to see if the auto-block would take effect then, and low and behold, it did!