I am sure you installed some security updates (like kernel updates) during the past 1.5 years that require a server restart, so it makes sense to reboot the server (or do you use some Kernel live patches?)
Based on the first screenshot, the server does not use swap (1GB) and uses 1.6Gb for buffers/caches (which could be quickly freed if required), which means there is enough memory. In this case, I would say if you see low free memory, it means the kernel uses it effectively and the memory does not idle.