TL;DR

Homebrew is removing --no-quarantine because:

Apple is killing Intel support.

Apple Silicon won’t run unsigned apps anyway.

Homebrew will soon require all apps to pass Gatekeeper.

They don’t want to help users bypass macOS security.

This is basically a security + future-compatibility cleanup.

> Apple Silicon won’t run unsigned apps anyway.

Technically true, but misleading. The macOS kernel won't execute an Apple Silicon binary that doesn't have a signature, but as Apple documents, an ad-hoc signature is enough to meet that requirement. That won't get you past Gatekeeper, but that's no different to how it is with unsigned Intel binaries.