enabling the HTTP/2 use by the HTTP client broke programs
connecting to servers with buggy HTTP/2 implementations.
These kinds of changes are unavoidable and
[permitted by the Go 1 compatibility rules](/doc/go1compat).
Even so, Go provides a mechanism called GODEBUG to
reduce the impact such changes have on Go developers
using newer toolchains to compile old code.

A GODEBUG setting is a `key=value` pair

pkg go/doc/comment, method (*Printer) Markdown(*Doc) []uint8 #51082
pkg go/doc/comment, method (*Printer) Text(*Doc) []uint8 #51082
pkg go/doc/comment, type Block interface, unexported methods #51082
pkg go/doc/comment, type Code struct #51082
pkg go/doc/comment, type Code struct, Text string #51082
pkg go/doc/comment, type Doc struct #51082
pkg go/doc/comment, type Doc struct, Content []Block #51082

pkg debug/elf, const R_RISCV_32_PCREL R_RISCV
pkg debug/pe, const IMAGE_FILE_MACHINE_ARMNT = 452
pkg debug/pe, const IMAGE_FILE_MACHINE_ARMNT ideal-int
pkg expvar, method (*Map) Delete(string)
pkg go/doc, const PreserveAST = 4
pkg go/doc, const PreserveAST Mode
pkg go/importer, func ForCompiler(*token.FileSet, string, Lookup) types.Importer
pkg go/token, method (*File) LineStart(int) Pos
pkg io, type StringWriter interface { WriteString }

pkg expvar, func Handler() http.Handler
pkg expvar, method (*Float) Value() float64
pkg expvar, method (Func) Value() interface{}
pkg expvar, method (*Int) Value() int64
pkg expvar, method (*String) Value() string
pkg go/doc, func IsPredeclared(string) bool
pkg go/types, func Default(Type) Type
pkg go/types, func IdenticalIgnoreTags(Type, Type) bool
pkg math/big, method (*Float) Scan(fmt.ScanState, int32) error

pkg encoding/json, method (*Encoder) SetIndent(string, string)
pkg go/build, type Package struct, BinaryOnly bool
pkg go/build, type Package struct, CgoFFLAGS []string
pkg go/build, type Package struct, FFiles []string
pkg go/doc, type Example struct, Unordered bool
pkg io, const SeekCurrent = 1
pkg io, const SeekCurrent ideal-int
pkg io, const SeekEnd = 2
pkg io, const SeekEnd ideal-int
pkg io, const SeekStart = 0