Are modern drives able to be low-level reformatted by an end user or is this strictly a factory OEM process?

can an end user these days truly low-level format a modern HDD?

No, because there is no ATAPI command in current versions that provide such capability (e.g. there are essentially only sector write and read commands). Also there is no seek command anymore.
Since the actual drive geometry (numbers of cylinders, heads, and sectors per track) is no longer known by the user (or anybody/anything external to the drive), track format and explicit seek operations should not be performed.

For sure, "low-level" format (i.e. write the track with address marks, IDs, and gaps) disappeared when HDDs started using zoned-bit recording. The "low-level" format became an endangered command when IDE drives fixed the sector size to 512 bytes.