After upgrading vCenter to 8.0 u2 or later, "vCenter Server Setting" > "Timeout settings" has been disappeared.
vCenter 7.0
vCenter 8.0 u3
VMware vCenter Server
This "Timeout settings" in vSphere client assumes to be used as a session timeout value for legacy vSphere client (known as C# client which discontinued at vSphere 6.5) not for current vSphere client. So this configuration settings has been removed on vCenter server 8.0 u2 or later.
This is expected.
This "Timeout settings" is just for legacy vSphere client (C# client) which had been already discontinued from vSphere 6.5.
For configuring session timeout settings in current vSphere client is described official documentation below,