YugiohAi VS YgoMaster

If its the same mod i use, it was just updated 3 hours ago, you can redownload it here https://github.com/pixeltris/YgoMaster

YGOMaster Offline Client (it's not currently up to date. You can update it yourself, but it will take a little knowhow, or alternatively you could run it alongside the online client and you would still have access to the screen that lets you build your own custom duels while also being constantly up to date. I cannot guarantee that doing so won't get you banned, 'though, so do so at your own risk. The offline client isn't connected to your Master Duel account in any way, 'though, it isn't even connected to the internet, so they wouldn't have a way of knowing to ban you for that, it's just if you run it with the standard client that can get risky)

It's for offline play against a computer opponent, but this project on GitHub has a bunch of features that can be tweaked, including the ability to craft "royal rare" cards of all crafting materials.

There was a recent post asking if buying Link Evolution was a good idea for those who liked the Solo Mode gates, and if you just want to play with the decks without going through the story bits, there is always the offline Master Duel project on GitHub.

After my last meme post, I was challenged to see if I could get seven Link monsters out. Following much abuse of my... thinking-stuff, I came up with this combination of cards to test against the CPU opponent that the offline project lets you play against.

As I quipped on a similar meme post, the project that lets you try out various cards against an offline computer opponent is perfect for when you want to play decks that you either cannot or should not play on your online copy of the game!

You didn't miss anything! Nexus Mods has options for Master Duel, focused on replacing files on a user's PC copy. (I also found an older copy of the project to play the game offline, which I used to arrange this silly setup you see, but the most recent version of that is on that project's releases page.)

Not sure what a proper "Summon Skull" deck would look like, though, nostalgia alone has not been enough for me to try to make one yet. Might see what I can put together with the GitHub project that lets you play the CPU offline, but definitely not spending materials in the game proper!

You are right that it is not official, this project on GitHub is what is being referred to.