LCSC is a generally acceptable distributor, if not a first-tier affiliated one, I'm not sure. I would be a bit curious where they're getting those from.
It seems to me, a lot of older parts either remain in stock, or even production, in foreign markets. Which shouldn't make much sense, these are global manufacturers and distributors. And it shouldn't be due to secondary markets (surplus and salvage), but I don't what all sources they pull from, and maybe they do that from time to time. I assume they're either a primary distributor, or the lion's share of their business is. But I haven't looked into it for sure.
Might be able to ask them about date of production and sample lot codes for their stock, and in turn contact the manufacturer to confirm such.
Or just check if onsemi lists LCSC as an authorized distributor.
There are many ways to convey to others, the cooperation, good faith, and trust, that given parties share; the independent verifier pattern (A asks B for information on C, A asks C for confirmation about that information) is such an example. A manufacturer listing their authorized distributors puts them in a liability position: it would reflect poorly on them if the distributor they had authorized, behaves badly, probably among many other things in their supplier agreements. (The agreements themselves, are private and proprietary, probably for good (internal) reasons -- marketing secrecy, etc., so they can't post their agreements in full to broadcast that cooperation.) Betraying that trust would most likely have them pull their products at that distributor, a very visible signal to customers. In either case, both parties are likely to have some representation about each other, that can be found on the website or by contact. It can take some digging to find things (or where and how to ask about verification like lot numbers and stuff, and some time to await their response) so poke around a bit and see. It's a good research exercise

Tim