I saw several delivery bikes parked on the sidewalks that all had the same thing: a big red tail reflector (of the same width as the rack) on the rear rack, which are usually just reflectors, but occasionally some of them were lighted from inside. Anyone familiar with this? Did they come with the rear rack?

One of the older Cateye tail lights was also certified as a reflector. I thought it was a useful feature, but it probably added $.10 to the cost, so it's been discontinued. Do any other taillights on the market have this?

Seems common on most battery and Dynamo Taillights I have particularly the European ones..
mine, A B&M 4D Toplight only came with the rack because the whole bike was built up by the Koga Miyata Company.
if you just buy a rack, it will not be included.

I have Busch and Muller Toplight taillights , on my Tubus Rack [2 horizontal holes 5cm apart]
the batery Cateye lights are also fine.. my TL 500 screwed on the usuual 2 vertical hole 2cm apart, brackets..

some others pop off to not be Nicked. clip says on.. people zip tye those on to not be road debris..

The Radbot 1000 tail light includes a reflector.

I have a 4D Toplight. It's a reflector and has 4 LED emitters- two in the center and one each pointing to the sides. No blink mode (as it's a German product and their laws prohibit flash/blink modes for all save service personnel).
I also have a Nite Rider Cherrybomb that has a built in reflector, but it is not as big as the 4D is.

Edit: Also of note is that the above light is not currently on the rack- it is mounted to the seat post at the binder clamp with this widget:

It's similar to this, about the same thickness, but completely flat without the middle coming out, also not rounded at the corners. It just looked like a simple large reflector integrated to the rack.
(I have radbot 1000, whose reflector is completely separated from the light.)

Might have been one of the Cateye Reflex series. Here's a link to one of them: https://www.cateye.com/en/products/detail/TL-LD560-R/

Not that one (sorry, but thanks for the reply ). One really cannot tell it's anything more than a reflector. No light bulb visible from outside. A completely homogeneous surface. I had seen them almost on a daily basis when passing by, but had no idea they were not that simple, until the other day when 1 or 2 of them were turned on.

It seems to be a good idea to save space (as opposed to separating the two features as in radbot). If only they could make the light brighter.