I'm planning a new AR pistol build but the last one I did was years ago.  At that time everyone was selling lowers with "pistol" serial number.  Is this still a thing or can I just use any lower so long as it is configured as a pistol?

Any never-'been built as a rifle' lower will do.

Any lower that was not initially configured as a rifle may be used to configure a pistol. Has ever been thus.

The only thing that has changed is ATF's formal acknowledgment in 2011 that if first configured as a pistol, the firearm may go back and forth between pistol and rifle.

- OS

Awesome.  Thank you.  I liked my set of Aero's and thought to get a couple stripped ones again.

The whole 'pistol serial number' or 'pistol' marked lowers were just a made up ploy by some lower manufactures to sell you something at an inflated price for which there was absolutely no legal requirement for those markings.  While makers knew there was no legal requirement for marking lowers with the word 'pistol' or using special serial numbers, they also knew the tin hat crowd would pay extra for such things if they could be scared a little.  All they had to do is float a few shill postings about bumpkin police officers not knowing their asses from a hole in the ground and that there was an ATF agent behind every bush who was also ignorant of the law then tell you how you could end up in club fed if you ran into such an officer/agent, and presto, they could charge you $30 or more extra for a lower all under the guise of keeping you legal and out of jail with your back-side virginity in tact.  And as Barnum said, there's a sucker born every minute and the lower sellers knew it.

Your state law will have more effect. IIRC some, or maybe one had a different 'registration' procedure for pistols vs rifles. Since odds are you will encounter a state or local LEO then being compliant in that regard will go much further.

At the Federal level the question has always been was the pistol first built as a pistol? If it was ever assembled and there is no proof of what it was made into - say, buying a used stripped lower at a gun show or FTF - then pass on it. Lowers are cheap and like the OP said, he's planning on building from new.

Best bet is to know for a fact, not somebody's word for it 'cause they aren't the one who will lose the trial. Small odds but shooters live (and die!) over stuff like that.