[suihkulokki]

Shim is signed with microsoft key, does it still require custom mode for booting?

[Nullabillity]

Yes. Shim requires that your kernel is also signed, and also disables support for kernel modules.

[xvilka]

And this shows why forcing that in shim was a bad idea. They should have made a version breaking the chain, and allowing to boot unsigned kernel.

[Nullabillity]

No, this shows why (forcing) Secure Boot was a bad idea. Allowing Shim to load unsigned kernels would be equivalent to having it bypass SB in the first place, which Microsoft would presumably never sign.