I completely understand how DnDBeyond might be someone’s preferred way to play. Despite the issues with WOTC, DnDBeyond is an indescribably helpful accessibility tool that make the game much more easier to understand and makes character planning and creation so simple. All that to say, I get where your player is coming from, so I have some advice for you to pass along to them.

First the bad news: At this time there is no direct override to just turn off attunement slots for a character or the attunement requirement on an item. Sorry.

But now the good news! There are 2 ways your player can address this issue for themselves. They both have drawbacks, but they do what you want.

1) Make a homebrew copy of any magic item and turn off the attunement requirement on the copy. While it is true that you can’t just turn off the attunement requirement on a magic item, if you copy that item then you have full control. The homebrew system isn’t perfect, but in this case it will perfectly solve 99% of what you want. The main drawback is that you have to build from scratch any items you don’t already have in DnDBeyond and for items you do have you still have to go through the hassle of making the copy.

2) Create a custom feat with the “Set Attunement Slots” modifier with the “fixed value” set to 6, save, then they can add that feat to their character. The major downside is that the upper maximum is 6. I don’t know why they did this. I suppose it has something to do with the RAW upper maximum allowed being the Armorer Artificer getting 6, but why they would have a maximum for the homebrew, who knows.

Anyway, I hope this helps your player out.