Nope, 3rd party controllers are still a thing. Companies like Mad Catz that were known for cheap and bad crap went under, but you can still
buy 3rd party controllers. It's just different companies making them now. The way console makers encourage you to buy THEIR controllers is the fact 3rd party ones may not have
all the features. For example, a
Power A controller for the Switch wont have motion controls, an NFC reader, "HD rumble," and it's not wireless. But it's also $40 or $50 cheaper then the Switch Pro Controller.
EDIT:
Logitech also got out of the racing wheel and flight stick markets. That doesn't mean nobody makes them.