well...i think it somehow still slows down a little bit....
my 7 stars player (under 21 yrs old) are all fast trainer... but sometime in training only gain max 3%....at the same time, my 18 yrs old ST and MC (got them this season) can get up to 7% from training... also... my 6 star ST (age 19) after the new training system already gain from Q65 to Q69 (119% already...which is very close to Q70...7 star ) but my all 7 stars players... so far just reach to Q72~
Also, all my first team are all 7 star before the new system...and played 90% of the matches so far in this season...and my sub some are 5 star and 6 star .... and I always train my full team (all 22 players) at the same time... so the 7 star not only gaining from the training but also most of the match...while my sub team only gaining from the training...but my ST Q 65 one still gaining faster than my 7 star
so, that can prove after reaching 7 star...they still slow down...even not as slow as before, but still...they slow down from gaining skill...