Do other vendors suppert highjacking/altering of DHCP options? I feel this kills the whole concept of DHCP.

I'm in for adding a CLI option to not request a default route for sure but the other stuff I do not comply. Please show e.g. Cisco / Juniper / Arista configs which support this highjacking.

This is an example scenario in which this comes handy:
You have three ethernet interfaces
Two are connected to different LANs
The third is connected to WAN (another router)
All networks offer DHCP and def gw
The LANs offer it for internet access
The WAN offers it for branch access
We want to keep def gw received for WAN and ignore internet access offered by those LANs

Only these three (gw, dns, ntp) benefit most to be hijackable. So I appreciate this point you mentioned and simplify the request to def-gw only if you agree.

Here below are attached images of GUI and text representation of a router which I use and supports this (it is Mikrotik. I currently have do not access to other brands to verify)

I can see a case where people deliberately do NOT want to use ISP provided DNS servers (to avoid DNS NX hijacking) (and/or lock to a major internet DNS server like google 8.8.8.8 or Quad9 9.9.9.9 or Cloudflare 1.1.1.1 for example)

set interfaces ethernet eth2 dhcp-options no-default-route

Implemented for gateway address

DNS domain name servers are always requested from the server but must be explicitly "allowed" by set systems name-servers-dhcp