The practice pre-dates the Internet by a few generations
Back in the days before instant worldwide communication, when there were such things as printed periodicals (called "magazines") there were some for firearms enthusiasts. Many of these now antique hardcopy had q&a sections where readers would write in asking questions about the age and origin of their individual firearms.
It became the accepted convention to xxx out the last few digits of serial numbers, to prevent problems. You see, in those days, with a complete description of the gun, including the entire serial number, people could make claims that "their" gun had been stolen, and claim insurance money, or just generally make trouble for the real gun owner.
Very few older guns had paper trails in those days, and fewer still were the people who actually had or kept the papers. If someone claimed that "your" Colt SAA, .45 cal, ser# 12343456 was actually their gun which had been stolen, and all you can say was that you bought it 15 years ago, at a swap meet, and no longer have the handwritten bill of sale (or that it was a face to face swap, cash for gun, no paper involved-and completely legal to do at that time), when the cops show up asking questions, who are they going to believe? Even if they accept your story, its still a hassle.
You see, back in those days, it could take days, weeks or months to verify any valid paper trail, and for millions of guns, there was no legal requirement for any paper trail, nor background check, fingerprint ID, or anything else. Buying a gun was the same as buying a hammer, or a ladder, or a book!
The practice has continued, and carried over to the Internet, not so much as there seems to be a real world need to protect the privacy of the gun owner, but because there is still a desire to do so. So, maybe its paranoid, and if you thinks so, go ahead and post the full numbers of your guns! After all, whats the worst that can happen?
remember you aren't paranoid if there really is someone out there trying to get you!
or, in modern terms, you could think of it as one more step to help prevent identity theft!