The + is allowed in URI paths.
You can check it yourself:
- Visit the URI standard.
 
- Check which characters are allowed in the Path component.
 
- Note that every non-empty path may contain characters from the 
segment set. 
- Note that the 
segment set consists of characters from the pchar set. 
- Note that the 
pchar set contains characters from the sub-delims set. 
- And 
sub-delims is defined to consist of: 
"!" / "$" / "&" / "'" / "(" / ")" / "*" / "+" / "," / ";" / "="
As you can see, the + is listed here.
(See my list of all allowed characters in URI paths.)
A prominent example of + in the path of HTTP(S) URIs are Google Plus profiles, e.g.:
https://plus.google.com/+MattCutts