Pretty much what the titles says. On 1.21.1 -1.11.0 (NeoForge) there are no longer particle effects on dropped items. Worked just fine on the previous version 1.10.1.
I've uploaded a couple pictures to show visually what I mean. Configs are the same for both. (the feature is enabled)

Are they always missing, or only missing after a few hours of play? They seem to work fine for me, could it be the shaders?

You are in fact correct. I loaded a brand new instance with the latest version and it worked perfectly. Since I'm playing on a modpack something changed in the last several versions must have caused some kind of conflict. I will try and reach out to the modpack devs and see what could have caused the issue.
To add on further to the problem as the mod is client side I add it to many modpacks that I play and on another one the latest version also results in no particles but switching to the previous ones works just fine. While this time no matter if I use the latest ver or the one before it still results in no particles. Very weird.

Edit: The issue was a config option by the mod Loot Beams Refork. In the config of Loot Beams Refork > Subtle Effects Config > Force Disable Item Rarity > False fixes it.

P.S.
On another note while troubleshooting myself I discovered that Solas Shaders removes the particles (the shader in the pics is BSL which works just fine with the particles).

Good to know about the Look Beams Reforked config, thanks for letting me know. Also, I tested Solas Shaders to try to figure out what the issue was, but I didn't experience any issues with the item particles. Could it maybe be something else, like another mod, interfering with it?

The issue with Solas occurred on 2.8c. It's fixed in yesterday's latest release 2.9.

It seems that all the problems causing this have been resolved, so I'm going to close this now