RFC 1473 is pretty clear on its applicability:

This document specifies the following group:

The PPP IP Group

The PPP IP Group contains configuration, status, and control
variables that apply to the operation of IP over PPP.

Implementation of this group is mandatory for all implementations
of PPP that support IP over PPP.

But (earlier) it also says:

The PPP MIB is organized into several MIB Groups, including, but not
limited to, the following groups:

o The PPP Link Group
o The PPP LQR Group
o The PPP LQR Extensions Group
o The PPP IP Group
o The PPP Bridge Group
o The PPP Security Group

Without clear references to where those groups come from. Since
PPP-IP-NCP-MIB imports ppp from PPP-LCP-MIB, I guess I
need to at least look at RFC1471. I'm not so sure of the others.

I also found RFC 1317 which shows PPP in a protocol stack with RS-232

The RS-232-like Hardware Device MIB is mandatory for all systems
that have such a hardware port supporting services managed
through some other MIB, for example, the Character MIB or PPP MIB.

But the objects defined in 1317 are numerous, don't seem particularly
useful to me, and at least some of them will be difficult to implement.

So, mandatory or not, what MIBs are commonly implemented on systems
supporting PPP?