There is some limited 16-bit support happening.

And AVR too: https://github.com/rust-lang/rust/issues/42450

Are 8-bit systems still being used for new development?

There's 8-bit MCU's all over the place. I tried to find some numbers for you with a quick Google. This 2014 Amtel report puts 8-bitters at $6+ billion a year. Even the 4-bitters are still making hundreds of millions a year. I include a bonus link on that if it perplexes you.

http://www.atmel.com/Images/45107A-Choosing-a-MCU-Fredriksen...

http://www.embeddedinsights.com/channels/2010/12/10/consider...

Far as future, Jack Ganssle of The Embedded Muse has a nice assessment of it summarizing various sides:

http://www.ganssle.com/rants/is8bitsdying.htm

Oh, absolutely. The most familiar to software engineers is the AVR used in Arduino but there are so many 8 Bit micros out there (literally 10's of thousands different device) and new ones are coming out every month. So plenty of new development going on.

There are a couple of orders of magnitudes more Intel 8051 derivatives out there than there will ever be AVRs, and they still have yearly sales in the 100s of millions.

And how many percent of this is for new development?

There are still 24-bit systems used for new development.

Microcontrollers? Absolutely.

Yes but those are unlikely to be networked. The nasty stuff is something that is powerful enough to be connected to the internet and too weak to run modern languages. Fertile fields for Rust.