Using the URL scheme for adding a calendar event to Outlook web:
Outlook will convert all spaces in the subject and body to plus signs. Converting spaces to %20 in the URL has no effect. This issue has been seen on up-to-date Chrome and Firefox.
The only solution I have found that works is to convert spaces to thin spaces (HTML entity:  , Unicode: U+2009). The URL encoding for thin space is %E2%80%89.
Outlook properly renders the thin space in the subject and body. No telling how this affects other Outlook functions, but so far it seems to work for me.