Any feature set past the most basic (IP Base) will have IP SLA. The version number isn't important as much as the feature set - IP SLA has been around for years.
Note feature set upgrades are not free - you need to purchase the license in order to have the right to use the software. The product data sheet lists all of the available feature sets. (See Table 9 here.)
There are some architectural changes in IOS from versions 15 onwards, where licensing has been introduced. On those 15.x.
The new generation products and IOS are shipped with a universal IOS image that contains all the features available for use on the routers. The specific feature set is activated by the use of technology package licenses such as Security, Unified Communications and Data.
So in them the default IP Base doesn't have IP SLA Enabled by default. Following is the IOS Packaging Model for 1900, 2900 and 3900 Series Integrated Services Routers which shows under which Technology package what features are activated and subsequently carried further if higher package is enabled :
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