Impossible to open article called ";" by this URL: "https://pl.wiktionary.org/wiki/;". URL "index.php?title=;" works so this is only in ShortURLs.
It occurs when semicolon is at the end: "https://pl.wiktionary.org/wiki/a;" opens "a" instead of "a;", but "https://pl.wiktionary.org/wiki/;a" opens ";a" (ok).
Possible cause/solution is make rewrite rule (Wikimedia only?) on server to allow this pattern.
Cannot reproduce. https://pl.wiktionary.org/wiki/; opens for me in Firefox 70. Which browser is this about?
Note that ; is a reserved character according to https://www.ietf.org/rfc/rfc3986.txt section 2.2. The older https://www.ietf.org/rfc/rfc1738.txt says that "only alphanumerics, the special characters "$-_.+!*'(),", and reserved characters used for their reserved purposes may be used unencoded within a URL."
See https://en.wikipedia.org/wiki/Percent-encoding#Types_of_URI_characters
I just realized that this was filed before T238285, and I should have closed the other task as duplicate. Sorry!