Quick reply: sure, it is extraordinarily harmful. Virtually all of macOS’ regular protections are disabled (or disable-able) in restoration mode.
Restoration mode is meant to permit you to carry out fundamental setup and repairs to your system, like reinstalling macOS, repairing or formatting the disk, and altering safety settings. It’s NOT meant to be a mode the place you’d run any non-Apple software program, and it is a actually dangerous concept to run something you do not fully belief in restoration mode.
In actual fact, the script you linked prints this message whenever you run it:
THIS SCRIPT IS DANGEROUS!
DO NOT BLINDLY RUN IT IF SOMEONE JUST SENT YOU HERE.
IT WILL INDISCRIMINATELY WIPE A BUNCH OF PARTITIONS
THAT MAY OR MAY NOT BE THE ONES YOU WANT TO WIPE.You might be a lot better off studying and understanding this information:
https://github.com/AsahiLinux/docs/wiki/Partitioning-cheatsheet
That mentioned, whereas the script is definitely harmful in that it might delete stuff you actually did not imply to delete, I do not see any signal that it does something malicious. It solely deletes issues, it would not set up something or modify something (apart from creating a short lived file of UUIDs that’ll vanish whenever you reboot).
So I do not assume you have to fear that it is compromised your system, however it might be a extremely good concept to have a backup (or higher but, two) of something you do not wish to lose.