Hard to find? Good heavens, what's hard about using a search engine? This is a trivial thing to look up, Google will find you the right page fairly quickly if you keep your searches simple and don't try check all 5 possible answers in a single search. Search for "What's new in SQL Server 20xx" for each relevant value of xx working backwards from the earliest release you know contained spatial data types (or from the newest of the 5 releases mentioned in the question if you don't know whether any of them have that feature), ignore the obviously irrelevant pages in the search results (for example titles containing "Reporting Services" aren't going to lead you to anything about a Database Engine new feature, or pages released too long before the release contents were finalised), and you will only need to look at one page per release to get the answer.
For me it was extremely easy, because the only possibilities were 2008 and 2005 (I didn't know whether it ws in 2005 or not, never having used it and only having looked at it very briefly to decide to reject upgrading to it and wait for something better). My first search gave What's New (SQL Server 2008) as the first hit so I went there, clicked on the second link in the list of subtopics on that page, "What's New (Database Engine)", and there was the answer.