I don't agree. K&R 2nd edition only goes up to C89, while C99 added tons of language changes which make C a much friendlier (and dare I say: safer) language (the two biggies are designated-init and compound-literals, but also tons of smaller things, like declaring variables anywhere, "for (int...", a proper bool type, fixed width integer types (uint8_t, ...) etc etc...) - I would even go as far to say that C99 is the most important C version and almost feels like a new language compared to C89 (after C99 the changes were mostly incremental though).

The 90s were the decade when C saw its biggest improvements, and K&R 2nd edition stops just short of that.

(the book is still a good read as an interesting historical artifact - it contains a lot of wisdom that goes beyond language details - but as a language reference or learning resource it is hopelessly outdated)

C17 is the best version to target. It is a bugfix version of C11, so the feature set is older and more widely available than the date suggests. C11 includes atomics which are necessary for multicore programming.

It is more like the old testament, relevant, but not the whole picture almost 40 years later.