The Fokker F100s, besides being an “orphan” design with the demise of the company in 1996, was driven out of service with US carriers in the early-mid 2000s due to uncompetitive economics vs. the A319 and new-generation 737 at the top of the market, and RJs at the bottom.
Johnny, we may need to consider a definition distinction between “regional airline” in the generic sense of any smaller-sized airline that only covers a limited market, and the industry-jargon meaning of “regional airline” as the feeder services that serve small airports or thin-volume routes with connections to the hubs of the major airlines.
This is significant in that from your list, the 737, A319/320 and the MD80s are considered “main route” airliners, for the purposes of the union rules and and codeshare contracts that regional affiliates have with the national carriers (e.g. Delta Connection, American Eagle, USAir/United Express, Air Canada Jazz, etc.) to determine who flies what flight (e.g., ATL-CVG on a CRJ = Comair/Delta Connection plane/crew; ATL-CVG on a MD87 = Delta plane/crew). This has raised a new issue with the introduction by EMB and Bombardier of 90/100 seat RJs, which puts them close to the capacity of an A318, 737-500, MD95(B717), or the old DC9-10/20.
My entirely unscientific sampling from airport-watching and airline-magazine/website reading has the following results WRT regionals:
Beech 1900 and derivatives
Small carriers doing small-volume small- or remote- airport routes still use DHC-6 Twin Otters, Britten Islander/Trislander and even Cessna Caravans and 414s, a-la the TV show “Wings”.
Bombardier CRJ (-200 to -900)
EMB ERJ 135-145
EMB EJet 170-190
Dornier 328JET (literally a Do328 with the prop nacelles replaced with pylons for hanging jets)
A version of the BAe146, the AvroRJet, is still made but I don’t know if it’s in use stateside.
When we expand “regional airline” to include any small capacity, limited-route-system airline, e.g. the late Midwest, or Spirit, my planespotting/reading has included 737 (various series), MD80/90/95, A319/320 and EJet190.
Just so happens, the rise of the RJs resulted in a drying up of the market for, and a displacement from US routes of, many just fine late-generation props, and now with the rise in fuel costs some carriers are looking to props again only to discover that production capacity and competition is way down.