The problems of using an unsupported compiler version are almost entirely centered around incompatible dependencies, and this will be solved when Cargo learns to resolve dependencies to only compiler-compatible versions:
https://github.com/rust-lang/rfcs/pull/2495
If you're happy to use an outdated compiler, you should be happy to use outdated dependencies too (either way, you can't force the entire ecosystem to wait for you).
LTS won't help here. For example, Rust 1.31 (2018) would have been a good LTS candidate, and it still doesn't work for you! The complaints you've linked to are about base64 using 8-months-old "LTS" 1.31 which is still too new.
6-month LTS won't solve the Debian problem, which has a 2-year cycle. If they don't align, that's a 2.5-year lag you're asking for. Even if Rust itself really committed to supporting 2-2.5-year old versions, you still couldn't convince everyone on crates.io to follow this.