I also noticed that the search sometimes did not show some chips - in my case audio grade OPs like OPA1642 and similar.
There are quite a few extra parameters and some may exclude some chips. Anyway the search seems to change from time to time so it may be already be different now. With older parts it may happen they enter wrong data in the lists. I don't think they intentionally skip the older parts - they may be just miss some properties in the database (e.g. package family, type (BJT,JFET,CMOS), price). Ideally they would put the old parts to the bottom of the list.
The search at the distributors is also not always that convenient. The main problem there is the way the supply voltage range is handled - a long list with lots of ranges (e.g. 3 - 12 , 3.3 - 6 , ... is not practical at all). In addition there seem to be quite some errors in those lists - it mainly gets obvious when you get parts shown that do not fit at all, but one would likely also miss some due to wrong entries. The big advantage at the distributors is that one gets parts from different sources including some of the less common ones (e.g. ST, Toshiba, Samsung).