Issues during an update or a system crash can corrupt the library.
If you are a modder or require offline emulation for legally owned games:
Legitimate games that run on Steam use steam_api64.dll (notice the missing "r") to interface with Steam’s APIs for features like:
regsvr32 Steam-api64r.dll
are popular for standard DLLs, experts generally warn against downloading DLLs from random third-party sites because they could contain malware like cryptocurrency miners. How to Actually "Fix" It