Speaking for myself, I don't know in which exact version of Rust the features that I use was first introduced, and I don't think anyone else knows that either if they use a lot of different Rust language features.

So if I were to add a minimum required version to Cargo.toml for the crates that I put out, I would for sure end up putting the current stable version that I am using myself.

In practice I think that is probably fine, but also a bit unfortunate in a way, that anyone using an older version of Rust, such as those using distro packaged Rust will see Cargo tell them they can't build and run my crate, because I set minimum version to say 1.40.0, and they are using 1.37.0, but actually I might happen to not be using any features beyond what was available in 1.35.0 or whatever in some random crate of mine, but I don't know that and I don't wish to test every change I make to my crates in a bunch of older versions of Rust also because that would slow me down considerably.

On the other hand, if the Rust Team had the resources for it, could the crates.io service be made to detect minimum version of Rust needed based on the features used? Probably a lot of work I would guess. Doubly so when considering different feature flags and other cfg switches.

Again, I don't know the plans you have for how to do it so maybe you are already planning to do it that way but, you guys sometimes run builds on basically all of the crates on crates.io, don't you? Would it be feasible that the crates.io service runs a background service that builds each crate with a few different versions of Rust and records the build results and possibly test results into the meta data for the crates on crates.io? I think that would probably require a lot of server time, and therefore likely cost too much to be possible but it would have been really cool if it was doable.