The mollusc Ostreida (Class Pelecypoda) loost its foot, but not byssus muscles.This muscle join their two valves.

In oyster: "Settling from a planktonic to a benthic habit, and the accompanying metamorphosis, mark the end of the larval stage, soon after which the foot and byssus are lost and subsequent attachment is by cementation which is mediated by the mantle." See details in: Biology of oyster, Edited by Brian Bayne (2017), Developments in Aquaculture and Fisheries Science Volume 41.

So the byssus is kept in some adult bivalves such as mussels, but lost in oysters after metamorphosis.

Have a look at Tom Waller's study of the larval development of Ostrea edulis:

https://repository.si.edu/bitstream/handle/10088/5340/SCtZ-0328-Lo_res.pdf?sequence=2&isAllowed=y

Thanks for your all nice reply!