OpenSSH uses this feature now and I plan to use it in the future
for new extentions (ciphers and auth methods). Banning this will render
widely deployed software non-compliant.
The use of user@domain in the secsh protocol is completely orthogonal to
its use in URIs or email. These names are used as internal protocol
identifiers, not in addressing. IMO this is much more elegant a
namespace than X-whatever.
The "stability" issue seems irrelevant too - there are no catastrophic
consequences if the domain name goes away. Remember that these are,
by definition, vendor or site-local extensions.