At a guess:
1. Because 'Kotlin's' in the possessive form may confuse some people into thinking Kotlin is an (owning) company and implicitly detract from JetBrains as a brand.
2. Derivatives e.g. Kotlinize? I'm less sure; this probably just comes from some catch-all legal document about protecting brands. Pretty sure derived terms like 'Kotlinify etc.' are bandied around on the Slack channel without any JB employees batting an eye.
02/24/2020, 10:44 AM
well, I'd say slack messages qualify as fully informal, thus there should be no problem with us saying it here. I'd guess this is more about blog-posts or Readmes of libraries, such things. I'd say "Kotlin's data classes are an improvement over java's verbose syntax" is pretty clearly not referring to Kotlin as any kind of company.... But I guess a "catch-all" rule needs to be overly generalized ;D