It's really hard to measure stale block rates right now.

Fibre and compact blocks have made propagation tremendously in the network, so now when a block is stale it's usually stale at the first or first couple hops.

As a result, the stale block doesn't propagate well (because it just makes it through zero to a few nodes before intersecting the wavefront of the earlier block).

If you want an even remotely useful figure you need to get the output from getchaintips rpc from a bunch of nodes spread all over the world.

Last time I collected stats I got data from a dozen nodes and I was still learning about more new stale blocks from the dozenth.