Just to clarify, provider-defined functions are coming in OpenTofu 1.7, along with e2e state encryption. Generally, I recommend not comparing version numbers of Terraform and OpenTofu post-1.6.
Implementing the e2e state encryption was non-trivial, and we wanted to make sure we get it right, so that's why the release took us a while. We also got a slight additional delay due to needing to handle the C&D letter OpenTofu got from HashiCorp[0], but that's all sorted now.
The beta for 1.7 however is coming out this week, with the stable release planned in the next ~3 weeks.
[0]: https://opentofu.org/blog/our-response-to-hashicorps-cease-a...