If there's:

Client <-(Ethernet MTU 1500)-> Router 1 <-(Tunnel MTU 1420)-> Router 2 <-Ethernet MTU 1500-> ··· <-Ethernet MTU 1500-> aur.archlinux.org

For TCP connection, the client will initiate the connection using a little bigger mss, which actually cannot be carried by the link, destination aur will agree and use the same mss. However, when aur is sending actual data used full of mss, since the link between router 1 and 2 cannot carry the packet, the packet will be dropped at router 2, and router 2 will send an ICMP message to aur telling aur "your packet is too big and I just dropped it, please reduce your packet size", and that is ICMPv6 Packet Too Big. This is PMTUD, Path MTU Discovery, as expected, aur will reduce packet size wisely using the MTU specified by the ICMP message, so the connection can go on.

However, aur is not responding to it, but still sending packets using the incorrect mss, so the client will never get the response, the connection cannot be initiated, so users will not be able to use aur.

Please see the attachments.