Switching frequency — inductor sizing call
At 150kHz the inductor ripple current is higher, so you need a larger inductance or accept higher ripple at the output. At 300kHz the inductor can shrink, but the gate-drive losses in the external MOSFETs double. The 150kHz setting also keeps the fundamental switching noise out of sensitive AM-band or low-frequency instrumentation bands — worth noting if this supply shares a board with an analog front-end.
The tube shipment is typical for prototype volumes and small production runs — reel quantities are available through distribution.
No end-of-life notification is on record.