I'm pretty sure that years ago, you could do a search just by inputting the AFN. what happened to that feature? can you bring it back?
You still can, but only in searching the Ancestral File. People in Family Tree don't have Ancestral File numbers. However, once you find them in the Ancestral File you can generally use the regular ID they have there to find them in Family Tree.
Go to Genealogies and choose the advance search, there you will see the option to search by the Ancestral File Number:
When you have done a search and found the result, you can see the Ancestral File information for the person:
At the top of the column, underlined here, is an ID number. Ignore colons and everything before the second one. You can take that ID number and search in Family Tree and usually find the Family Tree record for that person. For example, in this case I get:
Note the the Ancestral File ID is labeled as such and is at the bottom of the right hand column of Ancestral File information:
Since the Ancestral File has been frozen for many years, Family Tree will generally have better information. However the Ancestral File could still be useful in finding people that were incorrectly merged.