You probably know what I mean from the title. How to make a homebrew magic item rarity to unknown?

We cannot. The best we can do is set it to “variable.”

Quote from IamSposta >>

We cannot. The best we can do is set it to “variable.”

 This seems inconvenient to me, they should make Unknown.

There are no officially published magic items with an unknown rarity. DDB is contractually obligated to hold as close to the officially published content whenever possible. If WotC were ever to publish a magic item with an unknown rarity, then DDB would implement that. But since the DM needs to know the rarity, WotC will most likely never do that.

Quote from IamSposta >>

There are no officially published magic items with an unknown rarity. DDB is contractually obligated to hold as close to the officially published content whenever possible. If WotC were ever to publish a magic item with an unknown rarity, then DDB would implement that. But since the DM needs to know the rarity, WotC will most likely never do that.

 I thought I read in the IDRotF support thread that The Codicil of White was unknown item

I wouldn’t know, I don’t buy adventures, only sourcebooks.

There's now an unknown rarity, if it still matters 3 years after the post...

Shield of the Silver Dragon