I have nothing more to add to what was said at the conclusion of the other thread, other than to point out that it’s a bit peculiar to make a new thread when another was locked specifically because we had already said “no” so many times. 
But in case you mean this more in the general sense of how can I do it anyway (rather than please add it), then do have a read through that thread as there are a number of answers given (and I believe elsewhere). Indeed the ease with which it is possible to password protect your device, your account on that device, encrypt your data (and even the whole machine up to the boot loader in some cases) and down to individual files and folders within that machine, in a portable fashion—all of that is why we’ve never seriously considered adding this.
All of these methods have nothing to do with Scrivener though, so if you don’t find something satisfactory there, it’s a good idea to broaden your search for general purpose queries on the matter. The methods you would take to protect one Scrivener project are identical to what you would use to protect any folder of files, and there are dozens of software vendors and developers out there make tools for that, from free (even provided with the OS in the case of Mac) on up to very expensive.