What this RS-485 transceiver does on the line
The 130 mV of receiver hysteresis gives you clean signal recovery on a noisy factory-floor cable run, which is the difference between a reliable link and a bus that keeps dropping bytes.
TI has marked the SN65HVD21D as Last Buy. That means the manufacturer has scheduled the final production run — once that inventory is gone, the part is gone from the factory channel. If this transceiver is on your active BOM, you need to either secure your lifetime buy quantity now or qualify a replacement before the last-ship date passes. The scorch mark on a dead board tells you which part failed; the Last Buy flag tells you whether you can still get a real replacement through normal distribution.
The 5 V supply rail is standard for legacy RS-485 buses; if your new design runs on 3.3 V, this part won't work without a level shifter. The 8-SOIC footprint is a common one, so board respin for a swap is minimal if you find a pin-compatible alternate.
Ships in Tube form. The small outline keeps board area tight, which matters when you're squeezing four transceiver channels onto a multi-port serial card.