Hey !
I listed all the lowest underleveled who can be shiny (Gen1-Gen7 games). I used some data of this forum and added corrections and more precision in the locations.
https://docs.google.com/spreadsheets/d/11jWl_il3K_JIZPZBG_G3BLajI7PiuqjVLwzksfWwlwA/edit#gid=1366347110