Skin#makeSearchButton, originally and still existing as a BaseTemplate method, was the de facto method for generating a "search" and/or a "go" button for ages.
I won't even touch the issue of why deprecating and planning to remove that and its closely associated sister method makeSearchInput is a bad idea; but I will note that as illustrated by the attached screenshots, something broke presumably as a result of 0acc0b51511a6f80ef4ff2475e8ec12263791891 (T301723) which touched a lot of the relevant code paths.

Skins which used to use this method to output two search buttons ("Go" and "Search") or maybe just one (usually "Go") now output an unlabeled button, which gets labeled with whatever your browser's default text for an <input type="submit"> without a value="" specified is; for Firefox this default text is "Submit Query" so instead of a "Go" and a "Search" button, the users will see two "Submit Query" buttons.

I have a bunch of yet unmerged fixes to address the situation across no less than 12 third-party skins hosted on gerrit. Luckily the fixes were as simple as defining the correct value attribute, which...MediaWiki core used to do until recently.

Again, it's not like this was a particularly hard bug to visually spot: with any of the impacted skins installed, load a page in it and observe the search box area and its submit button(s). Simple as that.

Screenshot of the Anisa skin (a skin with just the "Go" button; written and maintained by @Isarra) before the patch:

Screenshot of the Anisa skin after the patch is applied:

Screenshot of the Gamepress skin, a skin with both the "Search" button and also the "Go" button, prior to the patch:

Screenshot of the Gamepress skin after the patch: