The article explains the information where boot-server option is deprecated starting in Junos OS Release 20.4R1
From 20.4R1 Junos does not need to have "boot-server" config because Junos can adjust its system clock without "boot-server" config.
The main role of "boot-server" config was adjusting/synchronizing time regardless of time difference.
You can delete "boot-server" config because it will not affect NTP when removed.
[edit system ntp] boot-server (address | hostname); User-re0 > Edit User-re0 # delete system ntp boot server (address | hostname)
Note: The boot-server option is deprecated starting in Junos OS Release 20.4R1.