I don’t know when the change happened, but a few years ago Google Maps suddenly got a lot worse for biking. It’s still pretty good, but there was a span of about 2 weeks where it noticeably got worse for me, going from nearly as good as can be expected, to ummm I’m gonna trust my own judgment about 25% of the time.
Even in cities Google is similarly terrible, and it's actually gotten worse over the last few years as more unsuitable routes are listed as "cycle paths". I was trying to get home through London and Google sent me about a km in the wrong direction so that it could route me onto a muddy canal bank. I found myself wishing that I could simply put it in car navigation mode but avoiding routes that aren't legal for bicycles, but that's not an option either. E.g. from Great Dunmow to Bishops Stortford, all you need is to go straight down the old Roman Road, but there's no setting for Google Maps that will send you there: in bicycle mode it sends you down a muddy track, and in car mode it sends you down the A120 where bicycles are legally banned.
(Not to mention that here in Japan it simply refuses to provide cycling directions at all).
It's just not good enough, IME. It's well worth taking ten minutes to install OSMAnd and get decent directions instead.