What about learning to understand English?

"writing mostly safe systems programs."

Do you understand what mostly means?

Ada and Modula-2 have multi-threading as part of their ISO/ANSI language standard.

Also you don't seem to know much about Ada given your GC remark, but here is some learning.

https://archive.fosdem.org/2016/schedule/event/ada_memory/

Of course there are still use cases where Algol derived languages are still unsafe, however those use cases are a tiny portion of what happens in C land.