I call it the dumbest thing, because by the time the UEFA U21 begins, all the players are already 22-23. It's different than the Uefa U19 (where the players are U19) to qualify for the FIFA U20 the following year (when the U19s are now U20s because a year has passed - that makes complete sense).
The UEFA U21 is different. For example, the 2021 UEFA U21 required the players to be U21 in 2019 when qualifying began (so by the time of the actual U21 tournament the players were actually U23). It's not a "U21" qualification for a "U23" in a couple years (for the normal aging process) like the UefaU19/FifaU20. The UEFA U21 is never U21 except for super early qualifying games two years prior to the tournament.
It's also not a U21 group qualifying for the Olympics when they will be U23s. It's literally U23s qualifying for a U23 tournament the following year, for which they are not eligible - so it's not a continuity thing. It's totally off. The 2019 U21 year cut off was 1996 (aka U23 in 2019). They qualified for the 2020 Olympics, where the year cut off was 1997 (U23 in 2020 plus 3 overagers). So players are qualifying for a tournament that they may not even be eligible for if they are the max allowed age.
It should be renamed the U23 (which is what it really is) and when qualifying begins two years prior, the only players eligible would be players who would still be U23 in two years (ie. current U21s). That's what Concacaf does - the Concacaf U20 tournament is really a U19 tournament because they use the age deadline for the following year when the Fifa U20 happens.