QuoteMy question is more so if this is the appropriate thing I should be installing?I don't know Ubuntu enough to say if you need the -dev version or if libsoup2.4 would have been enough already (installing the -dev version should have installed it anyway). But to answer, yes you needed to install it.
QuoteAnd what would I need to install for Arch/Steam Deck?Probably libsoup https://archlinux.org/packages/extra/x86_64/libsoup/
QuoteAnd maybe if this could be automatically done by the Lutris installer?I don't know how Lutris works exactly. It does appear to have a section on required binaries needing to be present on the system https://github.com/lutris/lutris/blob/master/docs/installers.rst#requiring-additional-binaries but I don't know if that also works for libraries.