This has always been one of the benefits of Heroku to me: I don't have to architect my application for a single vendor's environment. If I use a hosted Postgres backend, then even more so.
What Google should consider doing is making App Engine "classic" services like Memcached, etc. available to any service running across GCP. Flex is fairly different from classic, but I think there's some confusion generated from the "App Engine" name and the baggage (good & bad) it has gathered over the years.
Already being done. First was Datastore, now Cloud Tasks is in Alpha. Expect to see others (Cron, Cache, etc) come as well.
Shameless plug, but for Go, there is an unofficial third-party (Chromium team) datastore package that feature near-seamless AppEngine standard/flex datastore support: https://chromium.googlesource.com/infra/luci/gae/+/master
It currently couples to any hosted memcached service, but should be forward-compatible with AppEngine if/when they release a Flex-accessible cloud version of their hosted memcache.
"luci/gae" also features a fully-compliant in-memory datastore mock for 1000x+ faster datastore testing and uses a memcache implementation based on "qedus/nds" which fixes several consistency issues present in "ndb".
Many AppEngine standard services are accessible using the "cloud" versions of APIs/endpoints. You may have to do some more heavy lifting, although on Flex you get GCE self-authentication for free, and most "cloud" API packages know how to recognize and utilize this.
Memcache is apparently on the list to receive a cloud API, although it's been in the "pre-alpha" state for over a year now. See: https://cloud.google.com/appengine/docs/flexible/java/upgrad...