Yes, you can still embed an Enketo form in a iframe on a different website.
If you are using Central, embedding is off by default because it exposes you to cross-site scripting attacks. To enable this functionality, edit your odk.conf.template to whitelist the sites where you want to embed. For example, if you want to embed on https://example.com https://company.sharepoint.com, you'd make these changes.
# More lax CSP for enketo-express:
# Google Maps API: https://developers.google.com/maps/documentation/javascript/content-security-policy
add_header Content-Security-Policy-Report-Only "default-src 'none'; connect-src 'self' blob: https://maps.googleapis.com/maps/ https://maps.google.com/ https://maps.gstatic.com/mapfiles/ https://fonts.gstatic.com/ https://fonts.googleapis.com/; font-src 'self' https://fonts.gstatic.com/; frame-src 'none'; img-src data: blob: jr: 'self' https://maps.google.com/maps/ https://maps.gstatic.com/mapfiles/ https://maps.googleapis.com/maps/; manifest-src 'none'; media-src blob: jr: 'self'; object-src 'none'; script-src 'unsafe-inline' 'self' https://maps.googleapis.com/maps/api/js/ https://maps.google.com/maps/ https://maps.google.com/maps-api-v3/api/js/; style-src 'unsafe-inline' 'self' https://fonts.googleapis.com/css; style-src-attr 'none'; report-uri /csp-report";
add_header Content-Security-Policy "frame-ancestors 'self' https://example.com https://company.sharepoint.com";
I've filed an issue at https://github.com/enketo/enketo/issues/1354 so the demo site you linked to gets fixed.