Hey there, not to pile on more issues xD; just noticed this change while messing around in Single Player testing other mods and realized it must have happened when I updated to 1.1.0, as I hadn't removed any of the pieces prior to updating.
Quick test showed that's the case.

V1.1.0:

V1.0.0

Players on my server can't see each other's armor unless they equip it after they're both logged in and in the same dimension. After traveling through dimensions the cosmetics need to be re-equipped again to be seen.

After looking at the logs, I discovered and reproduced the crash in a newly created test server with the latest server installer. (0.5.0.30)

The start of the error that pointed me to test just your mod and Fabric API:
[01:05:12] [main/FATAL]: Mixin apply failed cosmeticarmorslots.mixins.json:PlayerEntityMixin -> net.minecraft.class_1657: org.spongepowered.asm.mixin.transformer.throwables.InvalidMixinException Unexpected NullPointerException whilst applying the mixin class: null

Here's the rest of the log if it helps:
https://pastebin.com/DXLfKGhE

Not sure if this is a bug or it's intended but after i modify Cosmetic Armor's ingame settings and exit the game or travel to other dimensions (no custom ones though), they reset to normal armor instead. Is this supposed to happen? Not a game breaker but making the mod save the setting for the future might be handier, at least for when we go travel between dimensions.

1.1.0-MC1.14.2

If you open a game to lan, the mod has no effect on how other players see you. Both clients have the mod installed.

How I see myself:

How the other player sees me:

Hello.
Tried to download and install your mod; everything went as normal and it appeared in mod menu but I could not find an entry in the controls section to open the cosmetic armor slots GUI.
I thought it might be conflicting with another mod, maybe fabric controlling specifically since that allows you to search controls, etc. But everything else custom was appearing fine, so I tested your mod in a fresh instance with only itself (1.0.0 from CurseForge), Fabric API (0.3.0+build.185) and the most recent loader/mappings (0.4.8+build.155 and 1.14.2+build.7) and had the same issues. , Pressing all keys on my keyboard did not produce the GUI, either. I did not however, try unbind default keys.
However, when adding Mod Menu; I can see that the mod is loaded, at least to the point that mod menu pretends it's there.

The log seems rather uneventful to me, but maybe it will be of use to you. I loaded into a world briefly to see if it would produce any out of place output but noticed nothing.
https://pastebin.com/0PtRLKj5

Let me know if there's anything else I can provide to narrow down the issue. Excited to see more well-known modded features like this brought to Fabric .

Items placed in the cosmetic armor slots are lost upon death if gamerule keepInventory is set to true. The items do not drop, and are no longer in the player's inventory or in the cosmetic slots upon respawning.

When I test this on my server, i frequently have items I equip to cosmetic slots reappear under my cursor as though I've clicked them, but they are also in the cosmetic slot. It's much like a type of server lag where you put an item in an inventory but it remains attached to your cursor for a second, except that the item both in the cosmetic slot and attached to the cursor. Disconnecting and reconnecting puts the item back in my inventory.
I have the mod installed on client and server with Fabric 0.4.8 build 158 on MC 1.14.4

https://github.com/Lemonszz/villager-hats/releases/tag/1.14-1.0.0

cosmetic-armor-slots-1.0.0.jar

Version: 1.1.3

Doesn't work with textures from datapacks... Reverts to the default leather hat texture
https://www.planetminecraft.com/mod/hats-v2-0/