The f= parameter in a viewtopic URL is not needed as all viewtopic URLs work without it and as the canonical URL format does not have it.
The f= parameter was required in order to display a user / visitor in the forum's "who is online" list:
Because of that, visitors coming from canonical URLs (without the f=) were not displayed in the list. This was known as the "ghost users" bug which has now been fixed:
Thus, the f= parameter is now obsolete and can be removed - and as a result we can have simpler looking and prettier URLs.