jQuery ajax essentially is just a wrapper to XMLHttpRequest.

With some effort, the relevant jQuery methods that act as polyfills could be extracted.

However, a minified version of core jQuery is < 20Kb. A very low technical debt trade-off to support 99% of browsers.

Market share might be a ridiculously small percentage overall, but it's never appropriate to dismiss compatibility outright.

Reasons to include inefficient code for compatibility reasons might include applications where maximum reach is valuable, or where the scale is so large that the "infinitesimal market share" percentage plays out into thousands of end users.

Because market share is only meaningful if you target consumers. If you write (internal or niche) web applications for e.g. schools, large companies, non-profits etc. legacy browsers can easily be > 30% of your targets.