Managed to find the root cause of the regression:

cat << EOF > minimal.d
enum IOMode { all}

struct blocking {}

interface InputStream {
	@safe:

	@property bool empty();

	@property ulong leastSize();

	@property bool dataAvailableForRead();

	const(ubyte)[] peek();

	size_t read(scope ubyte[] dst, IOMode mode);
	final void read(scope ubyte[] dst) { auto n = read(dst, IOMode.all); assert(n == dst.length); }
}


interface OutputStream {
	@safe:

	size_t write(in ubyte[] bytes, IOMode mode);
	final void write(in ubyte[] bytes) { auto n = write(bytes, IOMode.all); assert(n == bytes.length); }
	final void write(in char[] bytes) { write(cast(const(ubyte)[])bytes); }

	void flush();

	void finalize();
}

interface Stream : InputStream, OutputStream {
}


interface ConnectionStream : Stream {
	@safe:

	@property bool connected() const;
	void close();
}

static foreach (member; __traits(allMembers, ConnectionStream))
{
    pragma(msg, __traits(getOverloads, ConnectionStream, member));
}
EOF

with 2.080.1:

---
tuple(connected)                                                          
tuple(close)
tuple(empty)
tuple(leastSize)
tuple(dataAvailableForRead)
tuple(peek)
tuple(read, read)
tuple(write, write, write)
tuple(flush)
tuple(finalize)
---

with 2.081.0:

---
tuple()
tuple()
tuple(empty)
tuple(leastSize)
tuple(dataAvailableForRead)
tuple(peek)
tuple(read, read)
tuple(write, write, write)
tuple(flush)
tuple(finalize)
---

That's why the proxy-wrapping doesn't work.

See also: https://github.com/vibe-d/vibe.d/issues/2181

PR: https://github.com/dlang/dmd/pull/8456

(In reply to Seb from comment #0)
> I'm still trying to reduce vibe-core, it's a bit difficult because DustMite
> is pretty greedy and just removing the mixin methods would also lead to the
> attribute not being found.

The best way to reduce a regression is to invoke a working and broken compiler in the test script:

https://github.com/CyberShadow/DustMite/wiki/Reducing-a-regression-between-two-D-versions

Commits pushed to master at https://github.com/dlang/dmd

https://github.com/dlang/dmd/commit/d3f65c73d521f0279c57fec4aec09d03f49b4028
Fix Issue 19064 - [REG2.081] Vibe.d's InterfaceProxy no longer works

https://github.com/dlang/dmd/commit/678a445c5a275d7dace466970b48cb2961341cd1
Merge pull request #8456 from RazvanN7/Issue_19064

Fix Issue 19064 - [REG2.081] Vibe.d's InterfaceProxy no longer works
merged-on-behalf-of: Sebastian Wilzbach <sebi.wilzbach@gmail.com>

*** Issue 19076 has been marked as a duplicate of this issue. ***