apb protocol checker (assertions)?

Simple answer is that there aren't any available.

I think the protocol is too simple to really need a protocol checker; about the only requirement is that PENABLE is low for the first PCLK cycle of any APB access (the SETUP phase) when PSEL is high, and then PENABLE is high for the ACCESS phase which either ends in the 2nd PCLK cycle (for APB2) or when PREADY is driven high (for APB3 or APB4).

I guess you could also test that PSEL, PWRITE, PWDATA (and PPROT and PSTRB for APB4) remain stable throughout an APB access.

Other than that I can't think of anything you would want to test. You can have back to back APB accesses, so no need to test that PSEL goes low between accesses (again the PENABLE signal transitions would indicate when a new APB access starts for that scenario).

Were there any specific aspects of the protocol you thought necessary to test that aren't covered above ?

So not an extensive list of cases you would need to test for, and if you do want to check your design complies with the above "requirements", this would hopefully be simple enough to code in your testbench, so not really requiring a standalone protocol checker design.