Threads are in syllabus and according to me before diving into the topic of thread one must try to understand fork().... It will help
Here is the syllabus provided by IITM for 2019..You can see that they have mentioned thread, but there is no mention of System Calls so u can leave that. Moreover I/O is also removed from the syllabus from 2016 but File System is there..
Syllabus of OS
Processes, threads, inter‐process communication, concurrency and synchronization.
Deadlock. CPU scheduling. Memory management and virtual memory. File systems.
Link of Syllabus--> https://drive.google.com/file/d/1ebiBGoXqaqel_263WbxHtp2mlk7j3Z33/view