Not explicitly no. Implicitly, it would be the same as Debian, and I would argue we go for sane/common sense security defaults.
Is there a particular threat your worried about/want me to comment on?
For applications, Debian ships firejail.
So developing for Mobian is no different than developing for Debian (in fact then I don't need to use the Modem, I just do all of my development on a Debian Machine).
So this isn't a Mobian issue per se?
EDIT: I also checked, you can install PWAs via Epihpany (which is installed by default on Mobian).
https://wiki.mobian-project.org/doku.php?id=devices
PostmarketOS runs on more devices (and is a great Distribution!), but they also run a lot more devices through Halium (which is a compatibility layer).
So then out of curiosity, how are you all able to port to so many devices? Do you have a bunch of volunteers, or all most of the devices not too difficult to port to pmOS?
pmOS has really good tooling called pmbootstrap. They are able to use the downstream kernels directly (or a mainline kernel if the device supports it), and it generally isn't too difficult to get pmOS to boot with the downstream kernel. However, because of the downstream kernel, you may not get a lot of features, and will have to work to port the features to pmOS.
They also have device catagories, so if someone wants to see the status of a device, they can look here: https://wiki.postmarketos.org/wiki/Devices which bins the devices for a user to quickly understand how well a device runs pmOS.