Getting some pretty informal/uninformed answers from r/AskFoodHistorians, so crossposting here.
Shucking an oyster today is a challenge even with modern metal tools. How did people do it in pre-metallurgical societies? Instead, did they simply cook them open? Did they smash them, then clean the meat for consumption, losing all the liquor? Did they actually have tools capable of effectively shucking?
We have some good clues from shell middens - human-made piles of discarded shells. Generally, oyster shells in middens show little sign of being smashed - if weathering hasn't been too bad, most appear to be intact.
The usual method of opening appears to have been cooking, at least some of the time apparently by directly cooking them in a fire. Another method that will leave less trace on the shells (since they won't be exposed to the fire) is steaming or baking in pits, perhaps heated by fire-heated rocks.
Quaternary International 239(1-2) was a special issue devoted to shell middens. From this issue, the following papers note that some were cooked-in-the-shell:
Junko Habu, Akira Matsui, Naoto Yamamoto, Tomonori Kanno, "Shell midden archaeology in Japan: Aquatic food acquisition and long-term change in the Jomon culture", Quaternary International 239(1-2), 19-27 (2011). https://doi.org/10.1016/j.quaint.2011.03.014
A.C. Colonese, M.A. Mannino, D.E. Bar-Yosef Mayer, D.A. Fa, J.C. Finlayson, D. Lubell, M.C. Stiner, "Marine mollusc exploitation in Mediterranean prehistory: An overview", Quaternary International 239(1-2), 86-103 (2011). https://doi.org/10.1016/j.quaint.2010.09.001
Cooking shellfish open also appears to have been done by Neandertals: