Many folding bike companies have proprietary parts. Let's go through a few:

1. Brompton- nearly everything is proprietary, stem, frame parts, even the wheels are 349mm which is a strange standard ( 305 & 406 are more common ). This means replacing parts is will take time and money.

2. Birdy- This is one of my favorite folding bikes. It is light, comfortable, and stylish. However the fork, and frame are proprietary. Replacements must come from Birdy. They also use the strange 349 wheel size. Unfortunately, Birdy has problems distributing in the USA, this means sourcing parts will be difficult.

3, Dahon- They use 74mm wide front wheels to thin the fold. All replacement 74mm hubs & forks will go through them, since they are extremely rare parts. I don't think saving 26mm is worth the risk of having a bike with no parts sources.

4. Strida- Everything on this bike is proprietary, wheels, frame, belt, etc. These bikes are not too solid, but they are  a reasonable last mile solution for a lighter riders. I'm 200lbs and I don't think they can handle my size.

Overall try to stick with bikes that use Universal parts. Sourcing proprietary parts can be impossible ( especially during a pandemic ), so keep things simple with Universal parts that be sourced anywhere. FYI all of our bikes use Universal parts of the wheels, forks, etc.