Wrong version for enabling offhand swap.

[0-x-2-2]

ViaFabricPlus/src/main/java/de/florianmichael/viafabricplus/injection/mixin/fixes/minecraft/network/MixinClientPlayerInteractionManager.java

Line 282 in 6c8449a

if (ProtocolTranslator.getTargetVersion().olderThanOrEqualTo(ProtocolVersion.v1_8) && actionType == SlotActionType.SWAP && button == 40) { // Pressing 'F' in inventory

This was added sometime around 1.16.x, it works in 1.16.5 but not 1.15.2. Currently, this will send impossible packets detectable by anti-cheats however, this probably still works on servers that run native 1.16.5+ but the server has ViaBackwards and the client is using 1.12.2 protocol, etc with ViaFabricPlus.

[FlorianMichael]

Hello, thanks for reporting this!
After testing I confirmed that they added offhand swapping in 1.16, it's fixed now in the latest dev build on ci.viaversion.com

To address the ViaBackwards thing: In general VFP doesn't care about protocol translation being done by the server.

Have a great day!