Interesting that this came up.  I was just talking to someone else about this the other day.  From what I understand, that sysctl affects the max number of memory map areas a process can have, i.e. contiguous reserved memory.  For a game that's great because it can pack more into memory and decrease disk IO.  
My guess is that this is really not an issue for the Steam Deck because realistically it will only be doing a few things at once... Basic system, Steam Client, and the Game running.
How this would affect memory performance on a workstation or server with much more running concurrently, is something I can't really speak to, but I would guess there's a point where increasing it could cause issues depending on total memory available and how many processes are getting memory reservations from the kernel. 
At first glance it should be relatively easy to get at least a simple baseline check on a busy workstation/server that isn't massively loaded with RAM.  A system that starts to run into memory pressure during heavy use should reveal if this change causes problems under the same workload.