At the very end of 2016, all Lumia handsets had sold out in the Microsoft Store. The lack of an option to email a customer when they were back in stock implied that they wouldn't be; however, several models appeared anyway. As it stands right now, just the Lumia 550 and white AT&T variant of the Lumia 950 are in stock at the US Microsoft Store.
But now, it seems that the Lumia 650 and 735 are gone for good, as their listings have disappeared. If you head over to the Store's page for all Windows phones, you'll see that neither device is on there.
Of course, there are Windows phones that exist in the Store that aren't listed on that page, such as the unlocked Lumia 640 XL. Unfortunately, the 650 and 735 aren't even searchable.
If the Microsoft Store is going to drop two Lumias, it certainly made some odd choices. The Lumia 650 is the newest Lumia that exists, having come out almost 11 months ago. The Lumia 640 XL is a full generation behind it - being almost two years old - and that still has listings for both the unlocked and AT&T models.
The Nokia Lumia 735 is actually older than the 640 XL by six months, having been announced at IFA in September, 2014. In May, 2015, however, the handset finally found its way to the United States with Verizon Wireless as the Microsoft Lumia 735, and in fact, you can still purchase the phone from the carrier for $120.
Over on the other side of the pond, all Lumias were sold out in the UK Microsoft Store by mid-December and they haven't been restocked since. The only Lumias that you'll find on its page for all Windows phones are the Lumia 640 XL LTE, 650, 950, and 950 XL.
Naturally, if you really want any of these phones, you can get them from any number of third-party retailers, likely for a lower price than you would have from the Microsoft Store.