are you referring to this ? :
the Zenkai boost of Goku and Vegeta have already diminished to the point that it barely affects them anymore, but the series never stated that Zenkai boost stopped, neither the Guidebooks.
if Goku and Vegeta have reached their limits, then do we honestly believe that Saiyans have limits? The show proven numerous times, especially in the Tournament of Power arc, that Goku and Vegeta (
especially Goku) have broken their limits over and over and over again to the point where Goku break his own shell and achieved Ultra Instincts, while Vegeta ascended to the level of higher Super Saiyan Blue.
Even the theme of the upcoming Dragon Ball Super movie this December 2018, says that
A Saiyan has no limits, as they keep getting stronger and stronger due to their warrior blood, combined it with their biology to adapt and increase their power levels.