I think the weird thing about this tag is that discussions about what series should be tagged with yuri vs subtext are debating subtext vs romance, not subtext vs yuri. Why are the tags considered mutually exclusive? Seems more like a 4chan way of viewing the genre
I haven't seen the conversations, but I assume it would be because whether or not there is yuri is generally pretty clear.
On the other hand, it's possible for a budding slow paced romance to look like it's merely subtext at first glance. Especially if it's in a genre that have a habit of featuring subtext, like the oh so prevalent school girl slice of life.