Collecting information about getting postmarketOS running on Symbian devices. The important part is getting the Linux kernel running of course, if we can find one that run at one point (which probably does not exist?), then it's just doing the regular porting guide. Symbian was open sourced for one year, maybe that helps. Please add any helpful information or links that you can find.
Symbian was in fact made open source, at one point.
The final repositories from the Symbian Foundation may be found here. It may be of limited use, although the code has no doubt not been maintained for a long time.