That's not an English idiom that I've ever heard or read before. The lack of a "the" before "current" suggests it's a translation from a language that does not make compulsory use of articles. Perhaps it's an idiom in that language.
I've only ever read this on Hacker News, in the form "Welcome to $current_year!", where $ is the scalar sigil in the Perl programming language, indicating that it's a variable, and should be substituted by the reader as necessary.
Bracketing current year would have made it a little more comprehensible
I agree. That would have changed the meaning completely.