The warning is repeated forever, 1 per second, and pollues the logs 
without any added value compared to one warning for the whole stream.

Before:
$ ./ffmpeg -y -f lavfi -i testsrc=duration=10:size=8000x8000 -c:v ffv1 
-level 3 test.mkv
[...]
[ffv1 @ 0x560605332f00] Cannot allocate worst case packet size, the 
encoding could fail
[ffv1 @ 0x560605332f00] Cannot allocate worst case packet size, the 
encoding could fail
[ffv1 @ 0x560605332f00] Cannot allocate worst case packet size, the 
encoding could faild=0.0266x
[ffv1 @ 0x560605332f00] Cannot allocate worst case packet size, the 
encoding could faild=0.0399x
     Last message repeated 1 times
[ffv1 @ 0x560605332f00] Cannot allocate worst case packet size, the 
encoding could faild=0.0638x
     Last message repeated 1 times
[ffv1 @ 0x560605332f00] Cannot allocate worst case packet size, the 
encoding could faileed=0.0797x
     Last message repeated 1 times
[ffv1 @ 0x560605332f00] Cannot allocate worst case packet size, the 
encoding could faileed=0.0911x
     Last message repeated 8 timeses
frame=   17 fps=2.8 q=-0.0 Lsize=   15616KiB time=00:00:00.68 
bitrate=188126.9kbits/s speed=0.112x

After:
$ ./ffmpeg -y -f lavfi -i testsrc=duration=10:size=8000x8000 -c:v ffv1 
-level 3 test.mkv
[...]
[ffv1 @ 0x55acab29ef00] Cannot allocate worst case packet size, the 
encoding could fail
frame=   25 fps=2.5 q=-0.0 Lsize=   26368KiB time=00:00:01.00 
bitrate=216006.7kbits/s speed=0.101x