Antifriction

The late, great Sheldon Brown pointed out a dead-simple way you can squeeze an extra sprocket or two onto 7-speed freehubs using newer parts: just buy a 9 or 10-speed cassette and shifter, and take off one sprocket. The cassette then fits - and because the shift ratio for Shimano derailleurs was stable for 7 thru 9 speed (plus 10-speed road gear, but not mountain - as detailed in this deep dive from Art's Cyclery, which also goes into non-Shimano options), you don't even need a new derailleur. I just finished upgrading my 2009 Craigslist Eco3 to a 2x8 this way - making a custom 11-13-15-18-21-24-28-34 cassette from a mix of old & new cogs (viable, because in many cases changes in cog width are microscopic - though you definitely need the right spacers) - and it works fine.

However, your wheel needs to have a freehub rather than a freewheel. The Eco3 had a freewheel - I lucked into a freehub wheel at a closeout price last year, and was glad to get even a 7-speed one during the pandemic shortage. So - even though 7-speed folders are still being sold - maybe this is news for a niche that doesn't exist. Anyone know if 7-speed folders have ever come with freehubs?

willydstyle

From what I can tell most of the 7-speed folding bikes on the market are using freewheels with Tourney derailleurs.

Fentuz

My dahon helios D7 came with a 7 speed freehub. the helios was updated to 2x7 and then 2x9 and is still on the original sora short cage derailleur. I swapped the wheels because I found a cheap old "new stock" set of Kinetix comp for £60...
Tern b7 seems to be free wheel.
isla bike uses 7 speed cassettes on 130 hub.
https://cdn.shopify.com/s/files/1/04...f?v=1630577985

If run a cassette with a shimano derrailleur, you can upgrade the cassette to 8 or 9 speed and swap the shifter only. I have run Sora short cages and Altus M310 (sora is more precise than m310) in the following config:
7 speed cassette with shimano twistgrip 7s
7 speed cassette with shimano alivio trigger 7s
7 speed cassette with shimano tourney dropbar 7s
8 speed cassette with shimano altus trigger 8s
9 speed cassette with shimano sora trigger 9s
9 speed cassette with shimano acera trigger 9s
9 speed cassette with shimano alivio trigger 9s
9 speed cassette with shimano sora dropbar 9s

I run the following cassettes:
7s 11-30 (sunrace)
8s 11-32 (Shimano altus)
9s 11-25 (shimano )
9s 11-34 (shimano sora) - 2T over spec for m310 and 4T over spec for sora short cage
9s 11-36 (sunrace) - 4T over spec for m310 and 6T over spec for sora short cage
9s 11-40 (sunrace) - 10T over spec for sora short cage (not tested with m310)

Antifriction

Just found this, in a 2016 review of the Dahon Speed D7: "it has 11-30 Cassette instead of 14-28T freewheel that was used in its previous model." But it didn't last; the 7-speeds on Dahon & Tern sites at the moment (Piazza, Vybe, and Link A7) have 14-28, presumably freewheels. Still, there are production 7-speed freehubs out there, begging to become 11-42 9-speeds on the cheap.

Pinigis 

Most, but not all. The Origami Hawk is a 7-speed with a freehub.