I've updated the plugin to be more future-proof. It was reported the plugin didn't work for 1.21 but I could never confirm. I changed how the config.yml works to where any future additions to Spigot regarding the ItemFlag and Material enums will be handled without issue. With that being said:

I heavily modified the config.yml. You should take your current one (assuming you have any changes) and move it somewhere else and let the plugin generate the new one, then follow the instructions in the header of the new config.yml file to add your previous configurations. I shouldn't ever need to change the config.yml again in the future, even to support new features pertaining to the scope of this plugin.

I did very basic testing (put an item into my inventory and checked a Blacksmith Villager inventory) and the plugin worked on every version. If a bug is found, please post in it in the Discussion tab and I will fix it. Additionally, it should be noted that with the first experimental 1.21.2 release (which came out literally as I was updating the plugin for 1.21.1 lol), something changed with the Creative inventory and the plugin no longer automatically modifies items placed into your own inventory from the Creative menu. However, when you open up any other menu in the game or swap out of Creative mode or do literally anything else with the item, the item will be modified as intended. I do not intend to fix this as chances are it's probably not fixable and I doubt anyone cares that this problem exists as it shouldn't negatively impact anything feature-wise.

Requires at least Java 8. This will be the last functional version to require Java 8 (unless there is a bug with the current version). After this version, I will be doing what ProtocolLib is doing and moving up to 11 or 17 or whatever it was. However, this plugin works with all Java versions (up to 23 which is the latest at the time of writing). As such, Minecraft 1.8.0 will be dropped in the next version as well (1.8.8 will remain).

Requires ProtocolLib of at least version 4.6.0, but preferably the highest version your server can use. However, ProtocolLib dropped support for Minecraft 1.8.0 sometime after 4.6.0, so if you're on a 1.8.0 server, just use 4.6.0 (or go higher and see which ones still support 1.8.0). Version 1.8.8 and later can use the latest ProtocolLib version (5.3.0 at the time of writing) without issue. I did not test 1.8.3 or whatever the middle version is, so I don't know if it's the same as 1.8.0 or 1.8.8.