there is the table for xp gain modifier which modifies how much xp gains on certain level
the numbers as go are actually modifiers for the level so having
100, 80, 60 means that on firat level you gain 100% xps, on second level you gain 80% and on third level and higher you gain 60% of normal
so you can just fill it with numbers up to tenth lets say 50 and tenth number make 10 and your heroes will gain 10% of normal xp gain on level 10 on
this works for neutral creeps, idk if that works for player units too
the other way of doing this is triggering the xp gain by yourself
sorry for any grammar errors, Im on mobile right now