https://meta.wikimedia.org/w/api.php?action=help&modules=delete
pageid Page ID of the page to delete. Cannot be used together with title.
But https://meta.wikimedia.org/w/api.php?action=help&modules=undelete requires a title. It's much more convenient for language-independent tools to be able to use the pageid. So make it similar to delete which can use either a page title or pageid.
On second thought I may not need this due to other limitations. I thought I could maybe circumvent the need for T263209 (which nobody knows if/when it may be implemented) by following the suggestion from @Dipsacus_fullonum at https://meta.wikimedia.org/wiki/Community_Wishlist_Survey_2021/Admins_and_patrollers/(Un)delete_associated_talk_page.
But for my case, that won't work as the pageid of a deleted talk page can't be requested like that. The feature request remains valid (delete and undelete preferably support similar parameters), but there's no pressing need.
This is trickier than it seems, because a deleted page can have multiple page IDs, or (maybe) none at all.
It would also be nice to have a way to split possibly entangled page histories that are only separated by the page ID. That would be one solution to T71047 (or, well, not really - an attacker could clear that data by an extra undelete/delete, but it would help non-malicious uses of the same thing)
Hmm, maybe I should work on this ...
We're unlikely to work on this anytime soon. Marking this as Low priority, but if @Pppery posts a change, we'll try to review.