Why is `RUSTC_BOOTSTRAP` so actively discouraged?

FWIW, I think the issue mentioned in rust-analyzer is the kind of thing that does harm via propagating RUSTC_BOOTSTRAP. rust-analyzer apparently said "there's an unstable JSON output that we want, so we're going to set RUSTC_BOOTSTRAP to get it", and then a predictable set of problems ensued. This should never have happened in the first place; stable rust-analyzer should not be using this feature.