Still seems to work on some B450s (Asus B450-F and Asrock AB450M-HDV from what I tried, and not the B450-I or MSI B450 Carbon AC) actually, although the sale for $30 off is no longer shown on any of the B450 motherboards.
Same, not to mention most online retailers keep the record for you. For Brick & Mortars it only takes a minute to snap a picture or scan it to yourself for historical purposes.
QVL is a nice to have IMO, I haven't had too many issues especially with later BIOS revisions. The vast majority of us wouldn't notice the difference between DDR4 2933 @ CL16 and DDR4 3000 @ CL14 anyways. I suppose there is a concern with B350/X370 boards on old BIOS with CL16 RAM not even...
That's correct, they can go off serial number for warranty purposes (so you will be covered from the manufacture date, rather than the date of purchase). Better to get the receipt if possible :)