I had a quick google about this, and it looks like Apple actually removed support for some USB Bluetooth adaptors in High Sierra!
Which Bluetooth adaptor do you have? My guess is that it's one using a 'CSR' chipset. It seems that if you have one with a 'Broadcom' chipset (BCM20702?) then it should work. Some people seem to have reported success with a "Trust 18187" USB key.
Personally I'd contact Apple support - this is classic Apple, but IMO it's just not reasonable to remove support for previously working hardware when you update the OS.
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