For me, the big thing, and really the thing is music. I think that's something that's unfortunately really being lost these days among the newgen in the age of TikTok, 'starter packs' and streaming. Personally, I think it's well okay and fine to have a diverse music taste individually and to like other things outside of your niche (I always have) -- but for literal decades, these subcultures were DEFINED by their associated music scenes, so much so that many were literally even named after them (punks, goths, grunge, emo, etc.) that music set the entire tone, image, attitude, even the politics of the entire scene they were attached to.
So yeah, if you're claiming to be a 'tr00' Y2K (2000s) MySpace 'emo' and yet 90% of your playlist is Playboi Carti, Doja Cat and Kendrick Lamar -- and there's practically no Alesana, Funeral for a Friend, Thursday, or Saosin, or an traditional 80's goth whose "never heard" of such surface-level shit as Bauhaus, Sisters of Mercy or Siouxsie and the Banshees. It reads as incredibly suspect to me. Not because I have some vapid consumerist logic that dictates I think you need to exclusively listen/consume only one style of music and that's it or you're a fake -- but because without any dedicated exposure to that music, you're also 90% likely to be fundamentally missing everything else that comes with the music ideologically in the culture, and if you're just playing "dress-up" with other people's subculture because you "like the clothes/aesthetic" but do not even attempt to authentically engage with it's art or core ideological tenets... I'm sorry but that definitionally makes one a 'poser', or as one might say today a "LARPer".
Like, I get why this shit seems so dumb and surface level, the "purity-testing meme" of some random girl just wearing a Nirvana shirt she picked up at American Eagle/Forever 21/Urban Outfitters just because it looks cool and some sweaty 50-year-old loser passive-aggressively demanding her to "name five songs" over a piece of fabric -- but there is a genuine point to be made here. We wouldn't be so tolerant if people were brazenly doing the same degree of mindless appropriation for other's cultures (based on their religion, ethnicity, or country-of-origin) -- yet we deem it perfectly socially acceptable to do it for sub-cultures because we value them less for... seemingly no particular reason? In my mind, subcultures are just as valid to the cultural experience of a group of people as any other signifier (and that's ignoring that they can even extend beyond racial, geographical or economic divides).
It's also why I hate that starting around the early-2010s Facebook/Tumblr era, people started referring to a collection of post-hardcore/emo-adjacent artists (primarily PTV, SWS, BMTH, MCR, etc) as being this: "bands" (and not 'bands' as in literal bands, but as an established singular fandom named "bands") as if all other music is irrelevant outside the context of those handful that are accepted by the "bands" community. It's a fundamentally hyper-reductionist view of music. That practically kills the whole notion and point of music (esp. alternative music, which began explicitly as DIY counter-culture and not just 'edgy corporate pop') being to explore and find music that speaks to you, as an individual person, and find community around that -- and instead shovels them into a bunch of pre-approved record labels and corporate-marketing silos (Hello Vans, hello Hot Topic, hello MerchNow, hello Monster Energy, etc).