sorry I mean set like this

export CIN_10BIT_ENC=1

Now hevc_vaapi was able to render to yuv420p10le, that is 10-bit 420p, by selecting pixels p010le.
Also rendering with pixels y210 resulted in yuv420p10le, that is not 10-bit 422p as for hevc_qsv below.

I would assume this is caused due to the incomplete hevc_vapi.mp4 preset as shown below?

ffprobe -hide_banner cfhd01_hevc_vaapi_pix_p010le.mp4
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'cfhd01_hevc_vaapi_pix_p010le.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2mp41
    encoder         : Lavf61.1.100
  Duration: 00:01:11.20, start: 0.000000, bitrate: 11082 kb/s
  Stream #0:0[0x1](und): Video: hevc (Main 10) (hev1 / 0x31766568), yuv420p10le(tv, bt709/unknown/unknown, top coded first (swapped)), 1920x1080 [SAR 1:1 DAR 16:9], 11080 kb/s, 25 fps, 25 tbr, 12800 tbn (default)
      Metadata:
        handler_name    : VideoHandler
        vendor_id       : [0][0][0][0]

ffprobe -hide_banner cfhd01_hevc_vaapi_pix_y210.mp4
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'cfhd01_hevc_vaapi_pix_y210.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2mp41
    encoder         : Lavf61.1.100
  Duration: 00:01:11.20, start: 0.000000, bitrate: 11082 kb/s
  Stream #0:0[0x1](und): Video: hevc (Main 10) (hev1 / 0x31766568), yuv420p10le(tv, bt709/unknown/unknown, top coded first (swapped)), 1920x1080 [SAR 1:1 DAR 16:9], 11080 kb/s, 25 fps, 25 tbr, 12800 tbn (default)
      Metadata:
        handler_name    : VideoHandler
        vendor_id       : [0][0][0][0]

-----

No hevc_vaapi 10bit worked:

localhost:/Cin/ffmpeg/video # cat hevc_vaapi.mp4
mp4 hevc_vaapi
# cin_hw_dev=vaapi

I tested hevc_vaapi.m4 and tried to write p010 both in the pixels field and as format=p010 in the widget,
but only 8bit 420p each time.

-------------------------------

hevc_qsv 10 bit worked with p010 and with y210

localhost:/Cin/ffmpeg/video # cat hevc_qsv.mp4
# only usable with ext. ffmpeg, another pixfmt is yuyv422
mp4 hevc_qsv
# profile=main
# cin_pix_fmt=nv12
 

ffprobe -hide_banner cfhd01_hevc_qsv_pix_p010le.mp4
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'cfhd01_hevc_qsv_pix_p010le.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2mp41
    encoder         : Lavf61.1.100
  Duration: 00:01:11.20, start: 0.000000, bitrate: 28276 kb/s
  Stream #0:0[0x1](und): Video: hevc (Main 10) (hev1 / 0x31766568), yuv420p10le(tv, bt709/unknown/unknown, top coded first (swapped)), 1920x1080 [SAR 1:1 DAR 16:9], 28273 kb/s, 25 fps, 25 tbr, 12800 tbn (default)
      Metadata:
        handler_name    : VideoHandler
        vendor_id       : [0][0][0][0]


ffprobe -hide_banner cfhd01_hevc_qsv_pix_y210le.mp4
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'cfhd01_hevc_qsv_pix_y210le.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2mp41
    encoder         : Lavf61.1.100
  Duration: 00:01:11.20, start: 0.000000, bitrate: 32074 kb/s
  Stream #0:0[0x1](und): Video: hevc (Rext) (hev1 / 0x31766568), yuv422p10le(tv, bt709/unknown/unknown, top coded first (swapped)), 1920x1080 [SAR 1:1 DAR 16:9], 32071 kb/s, 25 fps, 25 tbr, 12800 tbn (default)
      Metadata:
        handler_name    : VideoHandler
        vendor_id       : [0][0][0][0]

you also can set bin/ffmpeg/encode.opts loglevel to debug, but render exactly one frame so log will be smaller.

How to render render exactly one frame ?

In render  dialog window there is selection of render range with 4 choices ... 1 frame mp4/webm should be perfectly legal :)

"git log" where?

in cinelerra-5.1 directory, or some down the hierarchy ...

this is command, part of git suite of commands.

displays log of commits in git repo. (for me it uses l"less" as pager, so you can scroll around and search)