I picked up some billet lowers because they have some of the newer features a lot of billet lowers have, a screw for the bolt catch and rear take down spring.
The set screw for the rear takedown spring and detent pin is a 3 minute modification that anyone can do, once they have a tap and set screw on hand (1/8" 4-40, steel, cup-point preferred). Run a 4-40 tap into the hole - start with a taper and finish with a bottoming tap, if possible - far enough to provide full-depth threads for the set screw, but not so far as to potentially cause spring binding. Blow out any shavings. Oil or grease the threads and parts, as desired.
Optional, but preferable, with a 1/8"-3/16" set screw; and mandatory with 1/4" or longer: Clip the spring by the length of the set screw. (Put the clipped end toward the set screw, if so.)
Install just a hair below flush with the receiver face, and you're done.
*Yes, 4-40 is slightly undersized for the hole. But it works fine.
Even though I rarely swap stocks or buffer tubes any more, all of my AR lowers have had the set screw installed, except for one. That lower was assembled as a pistol in 2014 and hasn't been messed with since.
The bolt catch screw would be nice. But once nearly anything is installed, it doesn't really matter.
There are alternatives. The AR world is very standardized and there are a lot of basic expectations - like using a roll pin for the bolt catch, or a shoulder pin if 'fancy'. But plenty of other options work.
I ran a polymer lower for about 2 years, which had a cotter pin holding the bolt catch in place -- because I had seen dozens of people saying that their 'ears' broke when the roll pin was installed in that same generation of receiver. I used a split pin made from a piece of drill rod in another, for the same reason and to allow for easy 'deep cleaning' while experimenting with some powder that was leaving a tremendous amount of unburned granules.
I have seen nails, machine screws, wire, drill bits, any other "scrap" items used for the purpose; as well as a guy that was using little keychain-sized clips ("carabiners") so he had a handy place to clip things.
ARs are adult danger Legos. Assemble and use however you desire.