@Legoktm and I have been working on a Rust library that replicates TitleCodec::splitTitleString. I just implemented IPv4 address normalization and noticed an inconsistency in how the software handles subpages of User or User talk pages.
If you try to visit or User:1.1.1.01 or User:1.1.1.01 , you're sent to User:126.96.36.199 because of normalization. Good.
If you add a slash, the server lets you view User:1.1.1.01 /, which says "User account "1.1.1.01 " is not registered." No IP address normalization.
If, however, you visit User:Example /, when the page User:Example exists, you get a subpage navigation link up to User:Example. Similarly, User:Example / test gives you a link back to User:Example. If you create User:Example / test and link to User:Example/test, it'll show you a redlink if User:Example/test hasn't been created as well. I demonstrated this on English Wiktionary: User:Erutuon / test. Spaces are significant there, even though the subpage navigation link goes to the same user page, whether the slash after the username is spaced or unspaced.
The inconsistency here is that User:Example / gets MediaWiki to recognize that a user exists by that name, but User:1.1.1.01 / doesn't recognize that you'd get a valid IP user page if you remove the slash and therefore doesn't normalize the apparent IP address.
A better behavior would be for TitleCodec::splitTitleString to remove spaces around the first slash in the User or User talk namespace title. A slash directly after the username is really just a directory separator, and there shouldn't be a distinction between the title that has a space before or after the first slash and the title that has no spaces around the slash. Such a distinction invites confusion.
Slashes later in user page titles on the other hand can legitimately be intended to mean "or", and spaces around them should probably be kept, even though they are interpreted as subpage separators by MediaWiki. It would cause frustration to remove them. Like perhaps someone would create a version of the English Wikipedia article Aoraki / Mount Cook in a user subpage (User:Example/Aoraki / Mount Cook) and they wouldn't want the spaces around the slash automatically removed (-> User:Example/Aoraki/Mount Cook). So it's probably best not to normalize away spaces around any slashes after the first.
This would be a breaking change, and quite a lot of userpage titles with _/ or /_ or _/_ directly after the username would have to be moved. For instance, this query of the English Wikipedia page table contains quite a few cases.