HDMI 2.1 is not required for 4K/60. HDMI 2.0 is, and any 4K TV will have that level. In fact, I just looked at the specs for the latest Roku Ultra (model 4800), and it says it comes with HDMI 2.0b, not 2.1. So I'm not certain what you read where.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." If you appreciate my answer, maybe give me a Kudo.