os.clock() does not appear in Roblox Studio Intellisense, and it does not highlight the member name as the other members of os does.
Current behavior, it does not show up on Intellisense and does not highlight member name:
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The behavior I expect, it shows up on Intellisense and highlights the member name:

To replicate this you open a script, type in os.clock() and observe what happens.
I do not know when this started occurring, I am currently on the latest version, 0.442.3.409454.