Please note: this document covers the changes from v6.12.6.

The main changes

support of JSON Schema draft-2019-09 features: unevaluatedProperties and unevaluatedItems, dynamic recursive references and other additional keywords.
comprehensive support for standalone validation code - compiling one or multiple schemas to standalone modules with one or multiple exports.
to reduce the mistakes in JSON schemas and unexpected validation results, strict mode is added - it prohibits ignored or ambiguous JSON Schema elements. See Strict mode and Options for more details
to make code injection from untrusted schemas impossible, code generation is fully re-written to be type-level safe against code injection.
to simplify Ajv extensions, the new keyword API that is used by pre-defined keywords is available to user-defined keywords - it is much easier to define any keywords now, especially with subschemas.
schemas are compiled to ES6 code (ES5 code generation is supported with an option).
to improve reliability and maintainability the code is migrated to TypeScript.
separate Ajv classes from draft-07 and draft-2019-09 support with different default imports (see Getting started or v7.0.0-beta.5 for the details).

Please note:

the support for JSON-Schema draft-04 is removed - if you have schemas using "id" attributes you have to replace them with "$id" (or continue using version 6 that will be supported until 02/28/2021).
all formats are separated to ajv-formats package - they have to be explicitly added if you use them.
Ajv instance can only be created with new keyword, as Ajv is now ES6 class.
browser bundles are automatically published to ajv-dist package (but still available on cdnjs.com).
order of schema keyword validation changed - keywords that apply to all types (allOf etc.) are now validated first, before the keywords that apply to specific data types. You can still define custom keywords that apply to all types AND are validated after type-specific keywords using option post: true in keyword definition.
regular expressions in keywords "pattern" and "patternProperties" are now used as if they had unicode "u" flag, as required by JSON Schema specification - it means that some regular expressions that were valid with Ajv v6 are now invalid (and vice versa).

Better TypeScript support:

Methods compile and compileAsync now return type-guards - see Getting started.
Method validate is a type-guard.
Better separation of asynchronous schemas on type level.
Type utility JSONSchemaType that generates the type for JSON Schema for type interface in the type parameter - it simplifies writing schemas (no unions support at the moment).

addVocabulary - NEW method that allows to add an array of keyword definitions.
addKeyword - keyword name should be passed as property in definition object, not as the first parameter (old API works with "deprecated" warning). Also "inline" keywords support is removed, code generation keywords can now be defined with "code" keyword - the same definition format that is used by all pre-defined keywords
Ajv no longer allows to create the instance without new keyword (it is ES6 class).

strict: true - strict mode
strictTypes: "log" - prevent mistakes related to type keywords and keyword applicability (see Strict Types)
strictTuples: "log" - prevent incomplete tuple schemas (see Prohibit unconstrained tuples)
allowUnionTypes: false - allow multiple non-null types in "type" keyword
allowMatchingProperties: false - allow overlap between "properties" and "patternProperties" keywords
loopEnum: Infinity - optimise validation of enums, similar to loopRequired
validateFormats: true - enable format validation
code: {optimize: number|boolean} - control code optimisation
code: {es5: true} - generate ES5 code, the default is to generate ES6 code.
code: {lines: true} - add line breaks to generated code - simplifies debugging of compiled schemas when you need it

keywords - now expects the array of keyword definitions (old API works with "deprecated" warning)

errorDataPath - was deprecated, now removed.
format - validateFormats: false can be used instead, old format mode can be chosen via ajv-formats package.
nullable: nullable keyword is supported by default.
jsonPointers: JSONPointers are used to report errors by default, jsPropertySyntax: true (deprecated) can be used if old format is needed.
extendRefs: $ref siblings are validated by default (consistent with draft 2019-09), ignoreKeywordsWithRef (deprecated) can be used instead to ignore $ref siblings.
missingRefs: now exception is always thrown. Pass empty schema with $id that should be ignored to ajv.addSchema.
processCode: replaced with code: {process: (code, schemaEnv: object) => string}.
sourceCode: replaced with code: {source: true}.
schemaId: removed, as JSON Schema draft-04 is no longer supported.
strictDefaults, strictKeywords, strictNumbers: it is default now, controlled with strict.
uniqueItems: '"uniqueItems" keyword is always validated.
unknownFormats: the same can be achieved by passing true for formats that need to be ignored via ajv.addFormat or formats option.
cache and serialize: Map is used as cache, schema object as key.