I’ve heard that they’ve been replaced by horizontal strobes or some other nonvertical strobe. Do vertical strobes still get installed?
Yes; I know this for a fact because the Simplex 4904-9138 strobe in still in production, along with the Edwards Integrity and one of its strobes as well. Even though it might start getting a little more challenging to find vertical strobes (especially on upgraded systems), the era of the vertical strobe is not quite dead yet. 
You “heard” wrong…
I still see a lot of Integrity horn/strobes installed outdoors on new systems. (The inside signals are usually the Genesis.) Also, when two of the dorms at Rutgers had new lounges built, they had Simplex 4903 speakers with vertical strobes just like the existing parts of the buildings.
To put it simple, vertical strobes aren’t illegal. Research has shown that an area can be better covered with horizontal strobes when compared with vertical strobes. They are still legal and still produced, as many people have mentioned, though you don’t really see any being installed except as replacements or to match alarms in the rest of a building.
My HS had the Simplex 4904 strobes installed in 2004 when they rennovated the HS offices and expanded the signal coverage.
Right. The MT-LSM series of Wheelock’s horn/strobes was replaced with the newer one due to the better coverage, but the MT-LSMs are still ADA-compliant, and if they had any new-old-stock, they could use them in new installations.
They also installed those MTL-style Faraday remote strobes at Massasoit College’s Student Center, replacing the original “FIRE” lights on the old Standard flush-mount retrofit plates on all but a few of them (one still has the original light and is located in a closet, and a few others got replaced with U-HNH horn/strobes.) They did this so the signals could be ADA-compliant and so the new strobes would still somewhat resemble the original lights. I have to admit, it looked rather cool that way. (The upgrade occurred in August 2007; however, one of the signals did not get the strobe upgrade until early 2009, and then a year after that, it was replaced with a U-MMT horn/strobe! I’m sure the old Standard 450 horn must’ve broken down or something.)