I have noticed the same. Funny enough, I found this post googling for "proxmox whixh interface vlan aware check box" (misspelling left intact) for this exact reason.
to everyone here i find that it does do something.
When you create a vlan network and add tagging to it, the vlan tag actually adds an additional header to the packets, so if the network is untagged the packets coming out dont have vlan tags (in networking we treat this as vlan 0 by default but it can be any value but this is just logical not in the actual hardware or l2 packets).
I set up a multi vlan and multi subnet network at home, after getting annoyed that spawning many ubuntu VM templates kept messing up the network. separating everything and giving the dev vm network its own large subnet helped a lot too. What would happen is my router would randomly not route VM packets so VMs would not have any networking (prior to vlan and subnetting).
The vlans are tagged on both the router and switch. I use a managed switch so the same wire passes multiple vlans to each proxmox server. When i put a VM on a tagged network and the NIC is untagged it doesnt work or expects the OS to have it set in its NIC Settings, so it does make a difference if you have network hardware configured as well whereas it doesnt make a difference if you are using vlans on proxmox only but not the switch. The difference between the checkbox is really just whether the guest OS should perform its own vlan tagging or proxmox for the VM. checking the box means that the vlan terminates at proxmox. My managed switch is fully managed and does L3 routing as well.
In my setup not only is the hardware configured with multiple vlans and subnets but its the same with proxmox. While i dont need to provide a subnet to every vlan on proxmox, each vlan has its own separate bridge. no bonding or bridging happens over multiple vlans (which may explain no difference to your situation). So if i have 4 vlans that means i have the NIC + 4 vlan interfaces on proxmox and 5 bridges on proxmox by default (if you add and dont remove the default config).
I've been spawning and deleting VM templates just to learn kubernetes and the network isolation is key to this, and i find the checkmark really does make the difference between having network access and not having network access if im using a tagged vlan prior to installing any kubernetes on the node itself. the big issue with network access comes when i use both ipv4 and ipv6 where i only get partial access (some sites load, some dont) but that may just be a router configuration issue, although ipv6 can be difficult depending on the ISP.
