Just to add a bit more detail...
In Advanced Search, you can choose to search just the text of the topic title, but that's not really a search by subject, just a search for the particular words used.
If you wanted to find posts about vices, for example, you'd miss posts headed "workholding options" or "old Record 52 versus new Irwin." You'd have no chance at all with threads just titled " help" or "newbie problem".
Some forum software allows users to tag their posts with what should be useful subject words, but that only works well if all posters join in.
Another idea, which I have seen elsewhere, is if some dedicated member has been in the habit of saving links to their favourite posts and then shares them publicly - but the effort of compiling such an index would be too great for almost everybody.