Dead end in pipes and legionella risk
I would not be overly concerned about this unless it was the highest point in your plumbing or if you had low water pressure from the water supply (or if you are on well water).
These couple of inches extra should be flushed with water at all times and recycled with the rest of the water supply.    There is no reason to believe that the water in these small sections would remain stagnant, as they are just way too small to have &quot;standing&quot; water.
If you are concerned I would have the plumber not cap these at all, take out the nearest T and change it for a right angle.
Now there are a TON of benefits to block framing (from the materials I am guessing France or Italy) but plumbing and rerouting the plumbing... no.   We simply don't have many instances in the US like this because the plumbing is easily modified inside 2x construction.