You mean address space. The OS can and will reclaim all the memory it likes, when it likes.
Pages that are mapped but are left untouched for a long time aren't problematic in modern systems. There is a small cost for the PTE but nothting like an entire page of physical memory.
I was talking about memory that was used in the past, but is no longer used.
The OS can either keep it resident or swap it out, but it has no way of knowing that it is no longer in use (short of something like madvise()). In the realm of sanity, the OS can't just arbitrarily throw away memory that a process has written to (unless it also kills the process).
Pages that are mapped but are left untouched for a long time aren't problematic in modern systems. There is a small cost for the PTE but nothting like an entire page of physical memory.