New Features (from CHANGELOG.md)

• Implement preview support for the @defer directive.
  @alessbell and @benjamn in #10018

• Implement useFragment_experimental hook, which represents a lightweight live binding into the ApolloCache, and never triggers network requests of its own.
  @benjamn in #8782

• Allow registering named fragments with InMemoryCache to support using ...NamedFragment in queries without redeclaring NamedFragment repeatedly in every query that uses it.
  @benjamn in #9764

• Support onError callback for useSubscription hook.
  @jeroenvisser101 in #9495

Improvements (from CHANGELOG.md)

• Delay calling onCompleted and onError callbacks passed to useQuery using Promise.resolve().then(() => ...) to fix issue #9794.
  @dylanwulf in #9823

• Replace concast.cleanup method with simpler concast.beforeNext API, which promises to call the given callback function just before the next result/error is delivered. In addition, concast.removeObserver no longer takes a quietly?: boolean parameter, since that parameter was partly responsible for cleanup callbacks sometimes not getting called.
  @benjamn in #9718

• Allow preserving header name capitalization when creating an HttpLink with createHttpLink({ uri, preserveHeaderCase: true }). Otherwise, header names are converted to lowercase to prevent case-sensitivity bugs.
  @MrDoomBringer in #9891

• Make queries with a pollInterval respect the no-cache fetch policy, instead of writing polled results into the cache.
  @MrDoomBringer in #10020

• Deprecate the onSubscriptionData callback in favor of a new onData callback for the useSubscription hook. Deprecate the onSubscriptionComplete callback in favor of a new onComplete callback for the useSubscription hook.
  @jerelmiller in #10134

Potentially disruptive (from CHANGELOG.md)

• The optional subscribeAndCount testing utility exported from @apollo/client/testing/core now takes a single generic TResult type parameter, instead of TData. This type will typically be inferred from the observable argument type, but if you have any explicit calls to subscribeAndCount<TData>(...) in your own codebase, you may need to adjust those calls accordingly.
  @benjamn in #9718

What's Changed (auto-generated by GitHub)

• Replace concast.cleanup method with simpler concast.beforeNext API by @benjamn in #9718
• Implement minimal useFragment hook by @benjamn in #8782
• Fix BatchHttpLink regression that silently discarded some pending queries by @benjamn in #9793
• Delay execution of callback functions to fix React errors by @dylanwulf in #9801
• Return most recent result diff from useFragment, update test by @alessbell in #10030
• Supports "no-cache" fetchPolicy with pollInterval by @MrDoomBringer in #10020
• (Release 3.7) Rename useFragment -> useFragment_experimental by @MrDoomBringer in #10075
• Adds @defer support by @alessbell in #10018
• Improvements to the hasDirectives utility function by @benjamn in #10082
• chore: omit previousResult, lastCompleteResult from options/result by @alessbell in #10100
• Support header name capitalization by @MrDoomBringer in #9891
• Add onError option to useSubscription hook by @jeroenvisser101 in #9495
• Avoid letting defaultDataIdFromObject normalize objects with nullish ids by @benjamn in #9862
• Allow registering named fragments with InMemoryCache to support using ...FragmentName in queries without redeclaring FragmentName in every query by @benjamn in #9764
• Fix various flaky tests before publishing @apollo/client@v3.7.0-rc.0 by @benjamn in #10118
• Update handler names for useSubscription hook by @jerelmiller in #10134
• Add changes from #10134 to CHANGELOG.md by @jerelmiller in #10148
• Release 3.7.0 by @benjamn in #9633

New Contributors 🎉 🙌 🥳

• @jeroenvisser101 made their first contribution in #9495
• @jerelmiller made their first contribution in #10134

Full Changelog: v3.6.10...v3.7.0