Suricata

This is pulled in by der-parser. This version of the proc-macro crate is also required for Rust versions < 1.45. 6.0.x has a MSRV of 1.41.

Is there a reason it's labeled as deprecated? Are there problems we should be aware of?

No reason given. I think its just to make it very clear the project is done and in most cases you don't need it anymore as the features it providers are now in Rust itself.