How big of an impact is it that there are several nodes out there still running on versions older than 16? After looking at a few different node scanners, I found nodes running versions as old as version 13 and some of these nodes have hash rates as high as 100TH/s.
The people running those nodes are hurting themselves, mostly -- but they can hurt others in two ways.
First, their hash rate is effectively lost to the v16 network, meaning the rest of the p2pool users wouldn't get paid if the old-version users hit a block.
Second, they can cause other users who are just rejoining p2pool to join their outdated chain, magnifying the problem. The majority network is now on v16, and rejecting any shares from v15 or earlier. There was a Reddit post a week or two back about a guy who spun up a new p2pool node with a slightly outdated software version (it didn't have the v16-only commit applied yet), got connected to the v15 network, and mined for quite a while wondering why he was getting poor results.
The upshot is that if at all possible, we should try to contact node owners still mining on v15 or earlier and convince them to upgrade.
. You just seem to know what you're doing. 
