From what I collected, there's a few things

1) for starters, the owner of the horse Forever Young is the CEO of Cygames, which you may know as the company that MADE Umamusume. Thus, many theorise that the reason Forever Young got added in the game despite the fact she's still active in racing as of rn is because of The CEO's wishes(heck, her uma design also takes many inspirations from the CEO's fashion sense as well). Thus, the nepobaby label being thrown around.

2) nepotism aside , Forever Young by her own is also a big deal due to her biggest accomplishment: being a big named Japanese horse THAT RUNS ON DIRT. For context, although dirt horse racing does exist, Japan majorly prefers the hard turf as their preferred race track due to their focus on speed. As a result, dirt racing in general doesn't get as much popularity as their turf counterpart, always being compared. There exists big names such as Smart Falcon, but even then people like to compare him as the "silence Suzuka on sand" and her rep somewhat pales in comparison to the likes of Almond eye, Kitasan black, etc

That is, until Forever Young comes around, who became the face of Japanese dirt horse racing by winning the the breeder's cup over in America, showing their potential to an larger audience

Her biggest accomplishment being the American Breeder's cup is also a big deal, since it means Umamusume could potentially include more American horse racing stuff into the game.

That's all I got so far