Are all US airline aircraft certified into known ice?
Title 14 CFR part 121.321 regulates scheduled airline operations in icing conditions.  It requires airframe ice protection systems in order to operate in &quot;conditions conductive to airframe icing&quot;, which they define as:

visible moisture at or below a static air temperature of 5 °C or a total air temperature of 10 °C, unless the approved Airplane Flight Manual provides another definition.

This essentially means that any airline that can operate in IMC is going to be subject to this requirement unless they have very niche operations at low altitudes in warm lattitudes where they can stay below the 10 C level when in IMC.
Aircraft operated by airlines that operate in IMC at temperatures colder than 10 C must either have an ice detection system or a means of detecting the first formation of ice on the airframe and then activate ice protection systems or they must, in the absence of detection, always operate their ice protection when operation in conditions conductive to airframe icing.
For smaller on-demand operations operating under part 135, 14 CFR 135.227 regulates flight into icing conditions.  This regulation requires that any airplance flying into forecast icing (IFR only) or known icing conditions (VFR or IFR) has functioning de-ice or anti-ice protection.  This means a 135 operation ferrying passengers from Miami into the Bahamas probably doesn't have to worry about this, but a IFR 135 operation anywhere it gets cold probably does.
For non-121 and non-135 operations in airplanes with 20 passenger seats or more, the applicable regulation is 14 CFR 125.221 and reads exactly like the part 135 regulation.
Summary
Combined, parts 121, 125 and 135 regulate the majority of commercial operations.   The requirements of part 121 realistically mean that any airplane suitable for scheduled airline operations is equipped with ice protection.  Part 135 can utilize a broader range of aircraft and while ice protection is not required it is quite limiting for the operation if it is not present (no flight into known or forecast icing conditions under IFR).
This means that all airline aircraft (part 121) are certified for FIKI but not all passenger airplanes (operated under part 125 or 135) are certified for FIKI (and cannot operate in those conditions if not equipped).
Regs

§121.321   Operations in icing.
(a) When operating in conditions conducive to airframe icing, compliance must be shown with paragraph (a)(1), or (2), or (3) of this section.
(1) The airplane must be equipped with a certificated primary airframe ice detection system.
(i) The airframe ice protection system must be activated automatically, or manually by the flightcrew, when the primary ice detection system indicates activation is necessary.
...
(2) Visual cues of the first sign of ice formation anywhere on the airplane and a certificated advisory airframe ice detection system must be provided.
(i) The airframe ice protection system must be activated when any of the visual cues are observed or when the advisory airframe ice detection system indicates activation is necessary, whichever occurs first.
...
(3) If the airplane is not equipped to comply with the provisions of paragraph (a)(1) or (2) of this section, then the following apply:
(i) When operating in conditions conducive to airframe icing, the airframe ice protection system must be activated prior to, and operated during, the following phases of flight:
...
(ii) During any other phase of flight, the airframe ice protection system must be activated and operated at the first sign of ice formation anywhere on the airplane, unless the Airplane Flight Manual specifies that the airframe ice protection system should not be used or provides other operational instructions.



§135.227   Icing conditions: Operating limitations.
(c) No pilot may fly under IFR into known or forecast light or moderate icing conditions or under VFR into known light or moderate icing conditions, unless—
(1) The aircraft has functioning deicing or anti-icing equipment protecting each rotor blade, propeller, windshield, wing, stabilizing or control surface, and each airspeed, altimeter, rate of climb, or flight attitude instrument system;
(2) The airplane has ice protection provisions that meet section 34 of appendix A of this part; or
(3) The airplane meets transport category airplane type certification provisions, including the requirements for certification for flight in icing conditions.