Unless it's obvious that oil has leaked out it's probably not dead, but if the car shows lack of rebound control at that corner, there may be a problem.
I experinced some strange behavior a few years ago at the RF of my Cosworth Vega. When I removed the Bilstein shock for investigation I could not compress it all the way, but there was no evidence of leakage. Took all the shocks down to Bilstein in San Diego and they rebuilt all four on warranty as I still had the original purchase receipt from 1979, and that was all I needed for them to honor the lifetime warranty on aftermarket purchases.
If you do turn out to have a bad strut, Bilstein should show an OEM replacement in their catalog, and as far as I know they still offer their lifetime warranty.
I ended up selling the rebuilt Bilsteins off the CV in favor of a set of Spax adjustables. I got a little more than I originally paid for them new.
A few years before one of the front struts in my 201/2.6 leaked oil and I replaced it with with a Bilstein OEM replacement. It was indistiguishable from the original, except the top of the strut hold was Torx instead of metric hex.