I just tested changing user agents in Chrome - setting UA to a random string like "fff" gets a search page, but setting it to "Links" or "Lynx" gets "Your browser isn't supported anymore" - which is to say, this doesn't look accidental, but more like these UAs are specifically being blocked.

EDIT: Forging user agent in Links (with links -http.fake-user-agent) gets a usable homepage, but the results page just sends you to a turn-on-your-javascript page.

My impression is that support for less common browsers and other unusual uses of google products has survived due to pet interest by individual googlers and not any sort of widespread policy.