Logitech Unifying is a 2.4Ghz radio technology that allows 6 compatible devices to be paired with one dongle.
While Bluetooth is a more generic solution, Unifying remains a broadly supported technology. It appears to be fully supported by Logitech, and you can search for Unifying devices on their website.
Logitech continues to sell wireless 2.4Ghz devices that don’t use Unifying. Most Logitech 2.4Ghz devices, however, come with a Unifying Receiver. A few devices come with a similar Nano receiver that does not unify, but the device can pair with a Unifying Receiver.
There is no comprehensive list of devices that connect with Unifying, but sellers of Unifying Receivers often include a list of many supported devices.
Logitech still sells Unifying Receivers for $15. You can purchase them used and new online from $10 to $15. There are also clone receivers from vendors in China and Turkey selling dongles for $7 to $10, but some people have reported problems online.
I have not yet purchased any of the dongles from AliExpress. What I’ve done, instead, is purchase Logitech mice with receivers. The mice will last a long time, but eventually fail in different ways. They only last so long. Then, you are left with a spare receiver!