We've had a number of bug reports in the past about extinct units showing up in the "Purchase Units" dialog even though "Disallow extinct units and parts" is enabled. Really it should only have said parts, because we had no way to determine whether a unit is extinct. Until now.
[Edit: as I continue to work on this, I see that there was some code in place that could make units unavailable based on the expiration of the part dates, but that was not as thorough and didn't provide for hiding from the list without a considerable performance hit.]
Draconis Combine, year 3000, with extinct units and parts disallowed, trying to find a Hussar. Almost any IS power would have the same result.
For contrast, here is the same search with all the settings the same except with ComStar as the faction:
Note that this only deals with the underlying components, not any fluff about availability of the units. So the Royal HSR-200-Db shows up in the ComStar list* because they have access to all the components, though according to the fluff there weren't any left behind by the Exodus. From the intro date up to 2810 and after 3040** both these units will show up for other Inner Sphere powers. If using variable tech levels you would have to have the maximum tech level set at least to advanced to see them before 3055, after which they become standard tech. Without variable tech levels they are considered standard because all the components are found in TW/TM.
*ComStar only gets different extinction rules if "Use faction intro dates" is enabled. Otherwise they are treated like any other IS faction.
**If using faction intro dates, other factions will have to wait a little longer, because ER Large Laser tech was recovered by DC in 3037 and doesn't become common across the IS until 3045.