I think Yesterday’s Enterprise Tasha Yar is still alive. hear me out. 

As the story goes, the Enterprise-C is destroyed at Narendra III and Tasha and a few crew members survived and are interrogated. A Romulan General becomes enamored of her and she agrees to become his consort if he’ll spare the life of the survivors, which he agrees to. a year later she gives birth to Sela and when her daughter’s 4 she attempts to escape but Sela cries out and they’re found out, and she’s executed.

Now my first guess is that the initial interrogation is done by the Romulan military, not by the Tal Shiar. Romulans have been in isolation for over 30 years at this point and might not think too much of Tasha’s weird new uniform and com badge so whatever. I also have to think their interrogation is cursory, all the Enterprise-C had left was Castillo - if he survived - a lieutenant and helmsman and a bunch of junior officers and enlisted crew. You’re not expecting much, just do it to say you did it. Tasha likely doesn’t break, and they’re not gonna bother corroborating if she’s actually in Starfleet with her name/rank/serial number. Maybe she jokes around she’s from the future but I don’t think she’s gonna call attention to herself. The whole point was to give hope for the future (and be with Castillo)

By the time this General struts around makes his move and offers the deal, the Tal Shiar probably gets wind of what happens, and being in intelligence circles and likely having operatives in Starfleet quickly find this Tasha anomalous. Depending on how Turkana IV is holding up by this time there may even be genetic records of lil Tasha there that sets a whole slew of alarm bells ringing (or even something as innocuous as Starfleet introducing new com badges a couple of years later). But this General, to be able to pull off his Tasha marriage stunt, is probably well connected for now that they can’t move against them. Sela is certainly factored in as possible leverage but the more they wait the more the information she does have could become out of sync with their time stream, but she would be seen as a potential treasure trove.

Here’s the fun part, I think they engineered the right conditions for her to attempt her escape. They might not have expected Sela to cry out but the results were the same, they were able to take her in, capture her. Interrogation with the Tal Shiar is a whole other level but they also want to keep her alive and refer to her from time to time and see how the information matches up as they get closer to her disappearance. But projects get defunded, priorities changed and people fall out of favour, and maybe a Borg attack on the Neutral Zone is enough to end the program and she’s quietly forgotten on ice. (Sela could have learned about her mother’s true origins thru whoever mentored her during her quick rise to power, or by digging deeper, or the fact that Prime Tasha Yar was on a high profile ship). And there sits Tasha Yar, in stasis, forgotten in some government lab or storage facility…just waiting for someone to stumble upon her years later…