There's now ngn/k[0] for a free implementation of K6. The author prefers K6 to Shakti, which is K9. And there are a number of other open-source K dialects and array languages as well[1]. For learning about K, the recently-created K wiki[2] is a great resource.
[0] https://codeberg.org/ngn/k
[1] https://aplwiki.com/wiki/List_of_open-source_array_languages
[2] https://k.miraheze.org/wiki/Main_Page