Due to the fundamental differences in architecture (Browser-based vs. Dedicated Client), it is not possible to play the real Rainbow Six Siege on an unblocked games website.
While not multiplayer, this series focuses on the patient, methodical nature of special operations, similar to the planning phase in R6. How to Play Official R6 Legally rainbow six siege unblocked games
For users on Chromebooks or devices where cloud streaming is not an option, "unblocked" sites often host lightweight clones or related tactical games: this series focuses on the patient