Note that formatversion=2 is still considered slightly experimental, in that backwards-incompatible changes like this can still happen. If you see any other changes like this that should be made, please report them!

Starting in 1.26wmf11, the following changes will be made in the output from meta=siteinfo when formatversion=2 is in use:

Output from siprop=namespaces will be an array, rather than an object indexed by namespace number. See T102645.[1]
The 'add', 'remove', 'add-self', and 'remove-self' subarrays in siprop=usergroups will always be arrays, never objects with numeric keys.
The (sub)arrays in the output from siprops restrictions, extensiontags, functionhooks, variables, protocols, and showhooks are now guaranteed to be arrays. Getting objects with numeric keys from these seems to have been unlikely or impossible anyway, but now it's guaranteed.

[1]: https://phabricator.wikimedia.org/T102645

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Brad Jorsch (Anomie)
Software Engineer
Wikimedia Foundation