project_id is from another era in phabricator land and does not provide any value.

r4nt · r4nt · commit 4a0cf284957f · 2016-09-27T15:47:29.000Z
Patch by Eitan Adler. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282494 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/.arcconfig b/.arcconfig
@@ -1,4 +1,3 @@
 {
-  "project_id" : "llvm",
   "conduit_uri" : "https://reviews.llvm.org/"
 }

Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,3 @@`
`1`	`1`	`{`
`2`		`- "project_id" : "llvm",`
`3`	`2`	`"conduit_uri" : "https://reviews.llvm.org/"`
`4`	`3`	`}`