Hello, 
I funnily asked chatgpt about your question, because you might understand it better than my previous answer about ISO-3166. Chatgpt answered:
In the context of OpenStreetMap, #LBY likely refers to Libya, which is a country code or ISO Alpha-3 code assigned to Libya according to the ISO 3166-1 standard. In OpenStreetMap, using the country code is a common practice to uniquely identify and tag features from different countries. So, #LBY may be used to tag various map features in Libya or to refer to projects or discussions related to mapping activities in Libya. Greetings