Most users should no longer import directly from lit-element, and instead prefer importing LitElement from the lit packages. The default entry point for lit-element remains backward-compatible and includes all decorators. However, it's recommended to use import {LitElement} from 'lit'; and import decorators from lit/decorators as necessary. See the Upgrade Guide for more details.
UpdatingElement has been moved from the lit-element package to the @lit/reactive-element package and renamed to ReactiveElement. See the ReactiveElement API documentation for more details. In addition, the source for css-tag, and all decorators have been moved to @lit/reactive-element. However, all symbols are re-exported from both lit and lit-element packages.
The @internalProperty decorator has been renamed to @state.
Errors that occur during the update cycle were previously squelched to allow subsequent updates to proceed normally. Now errors are re-fired asynchronously so they can be detected. Errors can be observed via an unhandledrejection event handler on window.
The lib folder has been removed.
Rendering of renderRoot/shadowRoot) via createRenderRoot and support for static styles has moved from LitElement to ReactiveElement.
The createRenderRoot method is now called just before the first update rather than in the constructor. Element code can not assume the renderRoot exists before the element hasUpdated. This change was made for compatibility with SSR.
ReactiveElement's initialize method has been removed. This work is now done in the element constructor.
The static render has been removed.
For consistency, renamed _getUpdateComplete to getUpdateComplete.
When a property declaration is reflect: true and its toAttribute function returns undefined the attribute is now removed where previously it was left unchanged (#872).
The dirty check in attributeChangedCallback has been removed. While technically breaking, in practice it should very rarely be (#699).
LitElement's adoptStyles method has been removed. Styling is now adopted in createRenderRoot. This method may be overridden to customize this behavior.
LitElement's static getStyles method has been renamed to static finalizeStyles and now takes a list of styles the user provided and returns the styles which should be used in the element. If this method is overridden to integrate into a style management system, typically the super implementation should be called.
Removed build support for TypeScript 3.4.
Decorators are no longer exported from the lit-element module. Instead, import any decorators you use from lit/decorators/*.
lit-html has been updated to 2.x.
Support for running in older browsers has been removed from the default configuration. Import the platform-support module to support Shady DOM. Note also that Lit parts inside <style> elements are no longer supported. See Polyfills for more details.
For simplicity, requestUpdate no longer returns a Promise. Instead await the updateComplete Promise.
Removed requestUpdateInternal. The requestUpdate method is now identical to this method and should be used instead.
#2103 15a8356d - Updates the exports field of package.json files to replace the subpath folder mapping syntax with an explicit list of all exported files.

The /-suffixed syntax for subpath folder mapping originally used in these files is deprecated. Rather than update to the new syntax, this change replaces these mappings with individual entries for all exported files so that (a) users must import using extensions and (b) bundlers or other tools that don't resolve subpath folder mapping exactly as Node.js does won't break these packages' expectations around how they're imported.

Minor Changes

A public renderOptions class field now exists on LitElement and can be set/overridden to modify the options passed to lit-html.
Adds static shadowRootOptions for customizing shadowRoot options. Rather than implementing createRenderRoot, this property can be set. For example, to create a closed shadowRoot using delegates focus: static shadowRootOptions = {mode: 'closed', delegatesFocus: true}.
Adds development mode, which can be enabled by setting the development Node exports condition. See Development and production builds for more details.

Patch Changes

#1964 f43b811 - Don't publish src/ to npm.
For efficiency, the css function now maintains a cache and will use a cached value if available when the same style text is requested.
Fixed reflecting a property when it is set in a setter of another property that is called because its attribute changed (#965).
Fixed exceptions when parsing attributes from JSON (#722).
Fixed issue with combining static get properties on an undefined superclass with @property on a subclass ([#890]https://github.com/Polymer/lit-element/issues/890));