Proposal: deprecate `@std/io`

[tomas-zijdemans]

Summary

I propose we formally deprecate @std/io: @deprecated tags on every export with a migration target, a notice in the module docs and README, and JSR archival once the one real migration gap closes. This isn't a new decision. When #5003 was closed, the stated plan was to stop stabilizing the package and "nudge people to use @std/streams instead", with a detailed follow-up issue to come. That issue never landed. Today nothing in the package tells users any of this.

Current state

• The package sits at 0.225.3 with no path to 1.0, per stabilize @std/io #5003.
• The module doc says "many of these utilities are also deprecated", but zero exports carry a @deprecated JSDoc tag. Editors and JSR docs surface nothing.
• JSR lists 131 dependent packages, so people keep adopting it without knowing its status.
• Piecemeal removal already started: StringReader (BREAKING(io): remove StringReader #6062), StringWriter (BREAKING(io): remove StringWriter #6061), and MultiReader (BREAKING(io): remove MultiReader #6059) are gone.

Migration targets

Export	Replacement
Buffer	Buffer from @std/streams
copy	readable.pipeTo(writable)
iterateReader	ReadableStream is async iterable
readAll	toBytes from @std/streams
toReadableStream / toWritableStream / readerFromStreamReader	Deno 2 resources expose .readable / .writable natively
writeAll	ReadableStream.from([data]).pipeTo(writable, { preventClose: true })
readAllSync / writeAllSync	none yet, see below

The sync gap

Web streams are async only, so readAllSync and writeAllSync have no replacement today. #6284 tracks the main use case (fully written sync output to stdout/stderr), and the suggested fix lives in the runtime, not std. I'd deprecate everything now, point the sync functions' @deprecated notes at #6284, and gate removal and JSR archival on that resolving.

Proposed steps

1. Add @deprecated tags to all exports with the targets above. Happy to send the PR.
2. Add the deprecation notice to the module doc and the README package table.
3. Once What's happening with writeAll and writeAllSync? #6284 resolves: archive on JSR and remove from the workspace.