`wgRelevantUserName` was [[https://gerrit.wikimedia.org/g/mediawiki/core/+/7127c1792bb9205d7313ffabb210963c485a2ebc|added to MW]] in 1.23, but it doesn't exist when viewing an IP range on (at least) Special:Block and Special:Contributions (compare [[https://en.wikipedia.org/wiki/Special:Contributions/192.168.0.2|192.168.0.2]] and [[https://en.wikipedia.org/wiki/Special:Contributions/192.168.0.0/16|192.168.0.0/16]]). Some awkward workarounds are possible but inconsistent (using `#mw-bi-target` or `input[name=wpTarget]` on Special:Block, `input[name=target].mw-input` on Special:Contributions); it'd be nice to have a unified and reliable way to get it, regardless of user type.
Of note: it seems to always exist on Special:DeletedContributions, and does so on Special:Log //iff// the range is given with the `user=` parameter (e.g., https://en.wikipedia.org/wiki/Special:Log?user=192.168.0.0%2F16).