Sure. But running an unmaintained operating system also is an huge security issue. I mean the proper choice is quite obvious: get rid of the phone and buy a new one that's supported by Calyx or Graphene. But since I'm not willing to do that, I have to choose what's more important to me.
And with security, it always depends on the specific threat model. I'm not sure if I need secure boot that badly. Can people steal my phone, flash a different OS and access the cryptographic key to my storage? Because that'd be one of the things I worry about. If not, I don't think I care about secure boot that much... YMMV