How are Polish diacritics entered these days? The sequence I found in Meta doesn't work

[supermathie (Michael Brown)]

This is definitely an OS-specific thing, not Discourse.

For unix-like OSes you can use the Compose key, e.g.:

Compose+e+' → é
Compose+n+~ → ñ
Compose+U+^ → Û

For Windows users an international layout with dead keys is probably the way to go.