If you want to use your system toolchain, just install your software from there, it will be compatible with the available toolchains, if it's a library, you don't even need the toolchain if its an application. The software is obviously also gonna be "outdated", because that's the point of Debian, stable software that has been well tested with the system.
> This is because Rust devs, for now, are the type that will immediately beginning putting the new code features
There is the concept of MSRV (Minimum Supported Rust Version) that a great fraction of the community follows. You just specify publicly the minimum version that your library supports and enforce it with CI. This is not as good as having an official standard (say, in Cargo.toml), but a lot better than just implicitly depending on what you think is currently available in bash for most users.
> without consideration that it's not available in any OS repository toolchains
Arch's package has been available on the same day in the last Rust's release. We'll probably have 1.63.0 available today or tomorrow.