I noticed this while testing RESTBase against Parsoid/PHP. I'm not sure whether the problem is in Parsoid or the MW router, but when requesting the page bundle for a specific revision of a title that contains a slash in it, Parsoid/PHP responds with a 404:
$ curl -H'Host: en.wikipedia.beta.wmflabs.org' https://en.wikipedia.beta.wmflabs.org/w/rest.php/en.wikipedia.beta.wmflabs.org/v3/page/pagebundle/User%3APchelolo%2FOnDemand_Test/275844 {"messageTranslations":{"en":"The requested relative path (/en.wikipedia.beta.wmflabs.org/v3/page/pagebundle/User:Pchelolo/OnDemand_Test/275844) did not match any known handler"},"httpCode":404,"httpReason":"Not Found"}
The same occurs when issuing the request for the html route, so this may be a problem with the MW REST router rather than Parsoid. Naturally, the page and revid exist:
$ curl -H'Host: en.wikipedia.beta.wmflabs.org' https://en.wikipedia.beta.wmflabs.org/wiki/User%3APchelolo%2FOnDemand_Test?oldid=275844 < HTTP/2 200 < date: Sun, 13 Oct 2019 11:07:35 GMT < content-type: text/html; charset=UTF-8 < server: deployment-mediawiki-07.deployment-prep.eqiad.wmflabs < x-powered-by: PHP/7.2.22-1+0~20190902.26+debian9~1.gbpd64eb7+wmf1
Fetching page bundles for pages that do not have / in the name works as expected:
$ curl -H'Host: en.wikipedia.beta.wmflabs.org' https://en.wikipedia.beta.wmflabs.org/w/rest.php/en.wikipedia.beta.wmflabs.org/v3/page/html/User%3APchelolo/347883 < HTTP/2 200 < date: Sun, 13 Oct 2019 11:11:38 GMT < content-type: text/html; charset=utf-8; profile="https://www.mediawiki.org/wiki/Specs/HTML/2.1.0" < server: deployment-mediawiki-parsoid10.deployment-prep.eqiad.wmflabs < x-powered-by: PHP/7.2.22-1+0~20190902.26+debian9~1.gbpd64eb7+wmf1
To make things even more interesting, if the page bundle of a title with / is requested without the revision, the request succeeds:
$ curl -H'Host: en.wikipedia.beta.wmflabs.org' https://en.wikipedia.beta.wmflabs.org/w/rest.php/en.wikipedia.beta.wmflabs.org/v3/page/html/User%3APchelolo%2FOnDemand_Test < HTTP/2 200 < date: Sun, 13 Oct 2019 11:14:38 GMT < content-type: text/html; charset=utf-8; profile="https://www.mediawiki.org/wiki/Specs/HTML/2.1.0" < content-length: 1242 < server: deployment-mediawiki-parsoid10.deployment-prep.eqiad.wmflabs < x-powered-by: PHP/7.2.22-1+0~20190902.26+debian9~1.gbpd64eb7+wmf1