The Premiere 2020 latest can not export 10 Bit with NVEC, only Software, is an very buggy Encoder. NVEC have 10Bit 420.
You have to use HEVC to get 10bit:
Actually, it is a hardware limitation of all Nvidia GPUs. None of them - not even the newest Ampere architecture - support 10-bit hardware encoding. Nvidia hardware NVENC is permanently locked at hardware level to 8-bit. No editing software can change that.
Plus, if you really intend to export as 10-bit H.264, you must use software-only encoding no matter which software you use.
"not even the newest Ampere architecture - support 10-bit hardware encoding" - Your information is wrong. It's works just fine if you use right software. Tested on nvencc: https://github.com/rigaya/NVEnc
Actually, I went by the official specs from Nvidia itself. Big companies like Adobe are bound to the official hardware company specs when implementing the encoders.
What I really meant to say that even Ampere officially supports only 8-bit H.264 hardware encoding.
From what I can see, OP did not asked about 10bit h264 specifically, only about 10bit hardware encoding in general.
I have an Geforce RTX 2080 Super it support NVEC 10 Bit, on the Davinci Resolve funtion this wit no Problem.
The Davinci Resolve can export NVEC H265 10Bit, the Geforce 2080 RTX have 10Bit NVEC, you will ony not programm this.
To the most recent post:
Adobe's hardware encoder has not changed significantly since it was first introduced with Premiere Pro 14.2. At that time, it was anticipated that most (if not all) H.264 and HEVC exports were 8-bit and 4:2:0. So, Adobe engineers at that time locked the hardware encoder to 8-bit-only. That has not changed yet.