Layer 2 (Ethernet) DCI will often be set up with 9000 (or larger) MTUs - the reason for this is to allow end-customers to carry their traffic using whatever encapsulation they want to internally (eg: Q-in-Q, MPLS, VXLAN, MACSEC, GRE or all of the above simultaneously), and the carrier/DC will transparently support it.
But when it comes to external L3 peering between 3rd-parties (eg: the Internet), it's very rare that people will mess with the IP MTU. Unless you can guarantee that the IP PMTU is higher than 1500 bytes (across multiple segments that you may not control, including ISP last-mile) there is simply no benefit.
But when it comes to external L3 peering between 3rd-parties (eg: the Internet), it's very rare that people will mess with the IP MTU. Unless you can guarantee that the IP PMTU is higher than 1500 bytes (across multiple segments that you may not control, including ISP last-mile) there is simply no benefit.