Possibly Shopify's monolith.

Or alternatively someone is doing something very silly.

Nah, they replaced GitHub with some in-house videcoded stuff a few months ago, and from what I hear from the grapevine it's not going well.

But there's a number of other big private repos with a high rate of merge on GitHub, and indeed it's currently a bit painful, e.g. the subroutine that check whether a PR can be merged tend to take ages. So my read is that they want to better support these big customers out of concern that they might leave too.