Pro/rel has been discussed ad nauseum here, just search and you can find all of the arguments for/against in any number of threads. IMO - it solves more problems that it creates, and on balance it would be helpful to add in the leagues that are currently operating without it (MLS N, ECNL, GA).
Formal/informal team-based pro/rel is in place in most all youth soccer, it's how teams go from bronze up through silver, gold, premier, etc. Every season a team that does really well tends to bump up a level, and in other cases a team that didn't do very well at all gets bumped down a level. It may not be a set "2 teams up, 2 teams down" formula, but coaches work to place teams in an environment where they can be successful.
This is common pretty much everywhere, up until the team (or player) reaches the highest levels of youth soccer, and once there - there is very little risk of the team ever being demoted/relegated. The leagues try to provide every justification for why this makes sense for them, which also keeps the process to remove poor performing teams completely opaque (and has everything to do with $, rather than performance).
Yes - it is also true that youth soccer at any level should never be optimized for most number of wins. It's not how to build younger teams, it's not how to run skilled teams, and even at the highest level - there is still a benefit for the club to build winning players rather than winning teams. Putting too much incentive on wins alone leads to poor outcomes, which is one of the reasons that MLS N has tried to not even count wins in the standings for some of the younger age groups. This concept of chasing wins, or taking shortcuts to wins, is often used to denigrate pro/rel at the youth level, but there is a difference between a team trying to get promoted, trying to be successful where they are at, and doing everything they can to avoid relegation.