Gaming support is still very much a work in progress all up and down the software stack. Stable distros like Debian tend to ship older proven versions of packages so their packaged software can be up to 18mo behind current releases. The NTSync kernel code that should improve Windows game performance isn’t even scheduled for mainline merge until the 6.10 kernel window in a few weeks - that’s not likely to be in a stable Debian release for a 12-18mo.
TL;DR: Gaming work is very much ongoing and Arch moves faster than Debian does. Shipping 12-18mo old versions of core software on the Steam deck would degrade performance.