Wow! I never even realised it had been going for that long!
I think that the reason that it isn't being made a big deal of is that there are different ways of counting it.
If we are talking about the number of times the Junior GP Series has been held, then last season's Junior GP Series was actually the 20[SUP]th[/SUP] edition.
If we are talking about years since the season the Junior GP Series was first held in, it is indeed 20 years this season.
If we are talking about calendar years since the first Junior GP Final, it will be 20 years in 2018 (although the Junior GP's were always held at the start of the season, the Junior GP Final was held towards the end of the season for the first 2 editions).
Confusing, ain't it?
So, it's not surprising that anniversaries for the Junior GP Series are not being made a big deal of.