Might be useful for someone, looks like RFC4638 is supported (tested in 4.3), the only thing you need to set MTU using CLI:
interface ISP ip mtu 1508
system configuration save
When PPPoE connection is restarted I am able to increase ICMP packet size up to 1472 (which corresponds to MTU 1500 minus 28 bytes header)
The only drawback is that UI settings show error for the field, if you need to change any setting on this page, it will not let you save it. The workaround is to reduce it to 1500 in UI, save, and change using CLI again. This could be something for Keenetic team to consider fixing (bump valid value to MTU size field to 1508).