Language changes

• Support Tuple#[](Range) with compile-time range literals. (#10379, thanks @HertzDevil)

Macros

• Don't use named argument key names as parameters for method_missing calls. (#10388, thanks @HertzDevil)

Standard library

• (breaking-change) Drop deprecated definitions. (#10386, thanks @bcardiff)
• Fix example codes in multiple places. (#10505, thanks @maiha)

Macros

• (breaking-change) Always add explicit return types in getter/property macros. (#10405, thanks @Sija)

Numeric

• (breaking-change) Change default rounding mode to TIES_EVEN. (#10508, thanks @straight-shoota)
• Fix downcasting float infinity. (#10420, thanks @straight-shoota)
• Fix String#to_f out of range behaviour. (#10425, thanks @straight-shoota)
• Implement rounding mode for Number#round. (#10413, #10360, #10479, thanks @straight-shoota)

Text

• Add missing unicode whitespace support to String methods. (#10367, thanks @straight-shoota)

Collections

• Fix Range#== to ignore generic type arguments. (#10309, thanks @straight-shoota)
• Make Enumerable#flat_map, Iterator#flat_map work with mixed element types. (#10329, thanks @HertzDevil)
• Remove duplicated sort related specs. (#10208, thanks @makenowjust)
• Fix docs regarding Set#each return type. (#10477, thanks @kachick)
• Fix docs examples regarding Set#*set_of?. (#10285, thanks @oddp)
• Fix expectation on set specs. (#10482, thanks @kachick)

Serialization

• (breaking-change) Serialize Enum to underscored String by default. (#10431, thanks @straight-shoota, @caspiano)
• (breaking-change) Use class instead of struct for types in XML module. (#10436, thanks @hugopl)
• Add YAML::Nodes::Node#kind. (#10432, thanks @straight-shoota)

Files

• Let IO::Memory not be writable with read-only Slice. (#10391, thanks @straight-shoota)
• Allow Int64 values within IO#read_at. (#10356, thanks @Blacksmoke16)
• Add IO::Sized#remaining=(value) to reuse an existing instance. (#10520, thanks @jgaskins)

Networking

• (security) Remove Cookie Name Decoding. (#10442, thanks @security-curious)
• (breaking-change) Remove implicit en-/decoding for cookie values. (#10485, thanks @straight-shoota)
• (breaking-change) Split HTTP::Cookies.from_headers into separate methods for server/client. (#10486, thanks @straight-shoota)
• (performance) Minor performance improvements to HTTP::Cookies. (#10488, thanks @straight-shoota)
• Respect subclasses when constructing HTTP::Client from class methods. (#10375, thanks @oprypin)
• Make the content-length header more RFC compliant. (#10353, thanks @Blacksmoke16)
• Fix #respond_with_status when headers written or closed. (#10415, thanks @straight-shoota)
• Fix Cookie#== to take all ivars into account. (#10487, thanks @straight-shoota)
• Remove implicit path=/ from HTTP::Cookie. (#10491, thanks @straight-shoota)
• Add HTTP::Request#local_address. (#10385, thanks @carlhoerberg)

Logging

• Close AsyncDispatcher on #finalize. (#10390, thanks @straight-shoota)

System

• Fix Process.parse_argument behavior against a quote in a word. (#10337, thanks @makenowjust)
• Add aarch64 support for macOS/darwin targets. (#10348, thanks @maxfierke, @RomainFranceschini)
• Add LibC::MAP_ANONYMOUS to x86_64-darwin to match other platforms. (#10398, thanks @sourgrasses)

Runtime

• Improve error message for ELF reader on uninitialized runtime. (#10282, thanks @straight-shoota)

Compiler

• (breaking-change) Disallow surrogate halves in escape sequences of string and character literals, use \x for arbitrary binary values. (#10443, thanks @HertzDevil)
• Fix ICE when exhaustive in-clause calls pseudo-method. (#10382, thanks @HertzDevil)
• Fix ICE when parsing foo.% calls. (#10351, thanks @makenowjust)
• Fix edge cases for symbol quoting rules. (#10389, thanks @HertzDevil)
• Support closured vars inside Const initializer. (#10478, thanks @RX14)
• Documentation grammar fix. (#10369, thanks @szTheory)

Language semantics

• Don't fail on untyped is_a?. (#10320, thanks @asterite)
• Fix named arguments in super and previous_def calls. (#10400, thanks @HertzDevil)
• Fix assignments in array literals. (#10009, thanks @straight-shoota)
• Consider type var splats in generic type restrictions. (#10168, thanks @HertzDevil)
• Align Proc.new(&block)'s behaviour with other captured blocks. (#10263, thanks @HertzDevil)
• Don't merge NamedTuple metaclasses through instance types. (#10501, thanks @HertzDevil)
• Access instantiations of NamedTuple and other generics uniformly. (#10401, thanks @HertzDevil)
• Improve error message for auto-cast error at Var assign. (#10327, thanks @straight-shoota)
• Exclude abstract defs from "no overload matches" errors. (#10483, thanks @HertzDevil)
• Support splats inside tuple literals in type names. (#10430, thanks @HertzDevil)
• Accept pointer instance types on falsey conditional branches. (#10464, thanks @HertzDevil)
• Match named arguments by external parameter names when checking overload cover. (#10530, thanks @HertzDevil)

Doc generator

• Detect source locations in more situations. (#10439, thanks @oprypin)

Others

• CI improvements and housekeeping. (#10299, #10340, #10476, #10480, thanks @bcardiff, @Sija, @straight-shoota)
• Update distribution-scripts to use Shards v0.14.1. (#10494, thanks @bcardiff)
• Add GitHub issue templates. (#8934, thanks @j8r)
• Add LLVM 11.1 to the list of supported versions. (#10523, thanks @Sija)
• Fix SDL examples crashes. (#10470, thanks @megatux)