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.