The Difference Between Deprecated, Depreciated and Obsolete

You are correct.

Deprecated means that it is still in use, but only for historical purposes and it will be removed probably in the next big release. It is recommended that you do not use deprecated functions or features - even if they are present in the current library for example.

Obsolete means that is already out-of-use.

Depreciated means the monetary value of something has decreased over time. E.g., cars typically depreciate in value.

Also for more precise definitions of the terms in the context of the English language I recommend using https://english.stackexchange.com/.

Records are obsolete, CDs are deprecated, and the music industry is depreciated.

In the context of describing APIs and such, "depreciated" is a misreading, misspelling, and mispronunciation of "deprecated".

I'm thinking people have just seen "depreciated" so often in other contexts, and "deprecated" so rarely, that they don't even register the "i" or lack thereof. It doesn't exactly help that their definitions are similar either.