MTU on router subinterfaces
The IP MTU of a router subinterface is only limited by the interface MTU of the parent interface. It is otherwise independent of the IP MTU configured on the parent interface. In the following example, the MTU of the parent interface is set to 1100, and the IP MTU of the parent interface to 1050. The allowed range for the IP MTU of the subinterface is shown below:
R1(config)#inter gig 0/0.1 R1(config-subif)#ip mtu ? <68-1100> MTU (bytes)
Therefore, the subinterface IP MTU is limited by the interface MTU of the parent interface.
Now as for the interface MTU, it is possible to modify this on the subinterface, but the router will always take a look at the interface MTU of the parent interface and the subinterface, and use the smaller of the two.
Links:
https://networklessons.com/cisco/ccie-routing-switching/pppoe-mtu-troubleshooting-cisco-ios
https://forum.networklessons.com/t/how-to-limit-the-mss-for-udp/16229/2?u=lagapides