Segments of the 99 are still in use and are called 99. I'm assuming you're referring to the segment of the 99 that went through the Tejon Pass. Parts of that Highway 99 are still in use and called various names but frequently called The Old Road. Parts of 99 were also used when they built Interstate 5, so having both the 99 and I5 would not be possible as they coincide for some stretches.

Not completely germane to your question but not unrelated is a piece done by Huell Howser about the original road that united the San Joaquin Valley and Los Angeles:

https://blogs.chapman.edu/huell-howser-archives/2003/09/25/ridge-route-road-trip-122/

Most of US 99 is very closely duplicated or overlain by I-5, and it's very rare for a US highway to remain a signed route when it only refers to short urban segments rather than giving cross-country guidance to motorists (the criterion used by AASHTO for such decisions). In addition, California was particularly aggressive in the 1970s about decommissioning any US routes that overlapped Interstate routes. That probably made a similar decision easier for Oregon and Washington.