I mean even in Halo 5 you could sometimes see some of the elite or grunt blood spurting out and onto the ground. There was also sometimes the weird orange yellow goo that would stain the ground from the Prometheans. That being said I do agree even when there was blood splatter it wouldn't persist in the world for too long. I think that comes down to 343 targeting 60 fps and them having to get rid of different effects on screen way sooner than many would want to try to maintain that 60 frames target. Hopefully with the Series X/S many of these effects will be able to stay on screen longer, although I'm sure on PC you could have them stay infinitely if you wanted to.
It is definitely the lack of persistent blood for the enemies that is the main problem. Here is a screenshot in Halo 5 immediately after a Warzone Firefight defense round. The bodies have already de spawned and the blood disappears as soon as you turn you back to it. https://imgur.com/DWTzTpt
The amount is correct (although I am not a fan of how muted they made the colors), but it needs to stick around longer. I think this is why people don't remember blood splatter in 5.
I do see the lack of any human blood at all as a bit off putting as well in the 343 games. With the return of Marines, there should be at least a little. That said, this isn't Gears of War. Gore should never been the focus, and really is best reserved for environmental storytelling/ evidence of a battle.