Using the information from this answer the last actual documented sighting of Roadster (2018-017A) that I can find is given in DASO circular 0567 on 2018/02/22.35017
Last observation that I can find documented is 22-Feb-2018.
While the other answer shows displays the date of an orbit solution, and in comments tries to link that to an estimated observation date, without any explanation of that, nor why it should be considered the last time Roadster was seen, at least my answer is sourced, which was stipulated in the original question for precisely this reason.
However, there is an explicit statement in Solution #10 of JPL's Horizons ephemeris for Roadster:
This trajectory is based on JPL solution #10, a fit to 364 ground-based
optical astrometric measurements spanning 2018 Feb 8.2 to March 19.1
Last observation that I can find mentioned but haven't found any documentation of is 19-Mar-2018.
So there should be a documented sighting around 2AM 19-Mar-2018 UTC reported somewhere! That doesn't mean it's the last, any observation after 27-Mar-2018 would not be documented in Solution #10, so this is not a good source for the answer. The dimming rate is slow enough that further observations are possible, just less likely.
Screen shots (click for full size):