Championed proposal: dotnet/csharplang#3630
Specification: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-11.0/required-members.md

Compiler

• Notify F#, C++/CLI, etc.
• VB (disallow attribute usage? tracking issue Required Members VB Support #61435)
• update test plan
• Review public API
• Any breaking changes? (parsing issues?)
• SyntaxNormalizer
• SymbolDisplay
• address PROTOTYPE markers
• local named "required" in top-level statements (C# 10 vs 11), will be handled as bug fix after merge (issue Local named "required" in top-level statements #61510)
• make type named "required" an error
• consider filtering RequiredMembers attribute in compiler layer/API (issue Consider filtering RequiredMembers attribute in compiler layer/API #61511)

declaration

• required as contextual keyword ("required" type, type parameter, etc exists in scope)
• where required modifier is allowed/disallowed
• disallow required modifier based on LangVer
• disallow required type name based on LangVer
• missing attribute type or ctor
• RequiredMember attribute emitted on type and members
• Overriding can tighten requirement
• Cannot hide required member
• Required member must as accessible as containing type
• RequiredMember attribute disallowed in source
• field/property without setter
• field/property with less accessible setter
• different kinds of members (field, property, indexer, event, nested type, ...)
• different kinds of containing types (class, struct, record; disallow interface)
• required with field/property initializer (allowed)
• rules with obsolete
• no definite assignment for SetsRequired
• must specify SetsRequired when SetsRequired on base or this
• combination with other modifiers (fixed, ref readonly, ref, const, static disallowed; others such as extern, partial, virtual allowed)
• record with required positional member declared explicitly
• modifier can be parsed in different orders

metadata

• Required attribute cannot be manually applied
• required members are marked Required and the type too. But not if merely derived from a type with required members.
• CompilerFeatureRequired with marker string
• Obsolete with marker string
• invalid type from metadata
• SetsRequired set on record copy ctor

usage

• object initialization
• nested object initializers new C { Inner = new D { ... } } versus new C { Inner = { ... } }
• attribute construction with named arguments
• new() constraint not satisfied

nullability

• nullable analysis of constructors in presence or absence of [SetsRequiredMembers] with required members
• initial nullable flow state of required setters, when the type contains some combination of required and non-required non-nullable reference properties.

LDM

• LDM: should we give a diagnostic for required property with an initializer (useless)? (answer: no, can be useful)
• LDM: should we do something about ref-returning properties (useless)? (disallow)
• LDM: should we do something about readonly modifier? (disallow)
• LDM: what if the required property is obsolete?

BCL

• Confirm we want types in BCL and complete API review
• Implement types (Add attributes for required members runtime#64287)
• Can/should Reflection APIs honor required?

Productivity:

• typing/completion
  • Partial doesn't trigger soft select in override completion #61439 covers soft-select during override completion, also broken for partial.
• override completion
• MetadataAsSource (done)
• formatting
• colorization
• F1/help (done)
• QuickInfo (P0: should not crash, P2: consider whether to show required) (no change, manually verified)
• Sorting modifiers (done)
• completion of object creation should call out required members
• snippet-style completion?
• some existing refactorings produce/modify constructors
• add option to generate required property for refactorings that generate properties