GPU acceleration unavailable in 2023 update

With Premiere version 23.1, the latest update, GPU acceleration (OpenCL) is available for me again.

I'm sorry about the misinformation on my part. The problem with GPU acceleration being disabled might have resulted from some OEM system manufacturers completely disabling all GPGPU support in their OEM-specific drivers, therefore rendering their discrete GPUs as display-only components. In this case, then the only way to restore GPGPU support would be to completely remove all traces of the OEM-specific Nvidia or AMD graphics driver, and then clean-installing the latest generic driver from Nvidia or AMD. You will lose all OEM customized optimizations this way. And unfortunately, if you try to restore the OEM customizations, then the GPGPU support will be disabled once again.

Big-name OEM gaming PCs - both desktop and laptop - are guilty of this.

Your Radeon RX 570 should work fine with Premiere Pro 2023, and OpenCL should be selectable as the Renderer.  That your drop-down menu shows Software Only as the only available option indicates that our "GPU Sniffer" background process apparently didn't discover your compute-capable GPU.  I've seen this happen on another system recently, where re-installing the AMD driver fixed the issue.  You might be experiencing a driver-related issue, too.  If a reinstall of the AMD driver doesn't resolve the issue, you might try running the DDU tool (display driver uninstaller, publicly available), and then installing a fresh copy of the latest AMD driver for your RX 570.  I believe either the Adrenaline or Pro driver versions should suffice. 

Premiere 2023 works with both OpenCL (on AMD & Intel GPUs) and CUDA (on NVIDIA).

In the screenshot below, I have both NVIDIA & AMD installed, and both CUDA & OpenCL show up as renderers.

(Of course, you can only pick one renderer at a time.)

OpenCL 3.0 is not required for Pr 2023.

I am sorry to tell you this, but the 2023 version of Premiere Pro now requires OpenCL 3.0 support in order to even use OpenCL GPU acceleration at all. Unfortunately, AMD chose not to support this newest version of OpenCL; its currently supported GPUs are currently restricted to OpenCL 2.1 support. Only Nvidia and Intel GPUs support OpenCL 3.0 at all.

Therefore, all discrete AMD GPUs will be permanently locked to the software-only mode unless or until AMD decides to add OpenCL 3.0 drivers to its graphics drivers - which is very, very unlikely since AMD has recently announced (silently) to completely discontinue OpenCL support (in favor of a different GPGPU API that Adobe does not currently support) moving forward.