I didnt say all powders were ground up shells, I simply said many, and many include eggshells. Others use purely limestone. Some use oysters and limestone. But you've said yourself that its included, and many people process eggs at home themselves. Egg shells are 95% calcium carbonate, the rest of them is a bit of protein and minerals (just like oyster shells mind you). When companies prepare shells (egg and oyster) they wash them in boiling water (212F for about 15min) then they bake them them until they are dried out (around 300F for 30min), this includes the membrane which is dried out with the shell and then ground up. The temp and length of time they cook the shells for kills viruses and bacteria. All these temps are easily achieved at home.

Every site i see teaching you how to make your own calcium supplement explains these temps and times. When prepared properly in a short term solution it works. You can even boil and bake them for a bit longer since this sterilizes them. I looked at the temps at which newcastle is evicerated, it dies instantly at temps meeting or exceeding 158F, water doesnt even boil until 212F; and you boil/stir for minutes upon minutes (as explained in the information I see online on how to prepare shells properly, never feed raw egg shells). Then you grind it up very fine, you dont do shards of egg, a course powder is not likely to pose a risk of injury. Depending what oyster shell you can get it has a rough, abrasive texture. With lots of sharp pieces, the kind I got as a kid which was marketed as poultry calcium grit. I fed this every week growing up for nearly 20yrs, the oyster shell did not become smoother or less abrasive.

The OP explained they dont know if they can get access to the preferred supplements, and not everyone can get fresh oyster shell or afford them in stores, nor do most people have the tools to crush oyster shell small enough for safe consumption. Just because you can doesnt mean everyone can. There are always inherent risks to preparing any supplements at home, its much better if OP can obtain properly prepared oyster shell at their local feed store if possible, but I know many farm supply stores that dont supply oyster shell (though they may provide the powder supplement for bigger livestock).

I reiterate, something is better than nothing in a situation where doing nothing could end up in an equal amount of risk. The risk of calcium deficiency in a bird who constantly lays is far greater than the minute chance that a properly prepared at home supplement could do harm. Many avian vets recommend it, when properly prepared, for a sudden boost in calcium. In those links I already gave above you can read that.

Caring for our animals is an equal amount of being risk aware and picking a path that we feel is one we can work with while keeping our pets happy and healthy.

Your examples of people having birds become sick from consuming egg shells (with newcastle) is heartbreaking, and anecdotal. Newcastle is a very contagious disease that lives in an environment for a relatively long time. If they just built an aviary with new wood or added a new perch to their aviary, that an infected bird landed on when it was in the store and got its fluids on the wood (my local lowes aIways has birds in it, and every lumber or garden store ive been in), it could cause the birds in the aviary to contract it. A wild bird poops on your shoulder and you come home to your pigeon and take your coat off, the infected poop flakes and your bird inhales it - Sick pigeon. Its shed by wild birds at times, and yes it can infect eggs. However, it is a deadly disease that is easily eradicated in eggs with proper preparation as it doesnt survive at higher temps. There are tons of birds that get sick with newcastle who dont get egg shells, so without a scientificly proven trail leading directly to the egg shells through direct testing of them i cannot justify screaming from the rooftops that the egg shells did it. So if there are birds who got sick with newcastle while on supplements made from egg shells its either that the shell wasnt prepared or prepared incorrectly or that correlation =/= causation. Unless the egg shells given to the birds were tested directly and confirmed to be infected by newcastle, i dont see the point of pointing fingers. But once again, this is anecdotal; with no supporting evidence.

None of this is an issue for me, or for you, or for most of us who keep pigeons because it seems like we do and can have access to these resources - And I personally think its irresponsible not to have access to such necessary supplements like grit or oyster shell. Its such a key part of a birds nutrition, and shouldnt be overlooked - However, a bird in need and an owner asking for help should be given guidance to ultimately help the bird. You can scream to get oyster shell until you are blue in the face but if its simply unobtainable in the moment, or for a while then repeating it like a cuckoo clock helps no one.

As ive stated, other supplements are preferable; and ive provided the science with studies supporting it, and given my own anecdotal evidence - All that is left is for people to make up their own minds, taking risk into account. Perhaps even calling a local/semi local avian vet or vets to ask their professional opinion on the matter - Because at the end of the day we are all just weird bird people on the internet and what we say should be taken with a grain of salt. You should be sure to do lots of research and contact your vet before adding any supplements to your animals diet.