I live in a high-altitude part of New Mexico, where I don't need to use much A/C, but need a more efficient heater for the winters.
Then you're looking at the wrong figure.
The number you're evaluating on is SEER, which is only relevant when the unit is in air conditioning mode. It says nothing about its heat performance.
For heating, the "dumbed down" number is HSPF, but what you really need is a table of COP @temperature. Because you need to make sure your heat pump will actually heat at your worst scenario temperatures.
COP is Coefficient of Performance, or the ratio of "watts of heat created per watt of electricity consumed". Your resistive electric heat has a COP of 1.00 by definition.
With a heat pump it will vary by source temperature. If you beat COP 2.5, it is actually more carbon efficient to burn natural gas in a power plant and run your heat pump, than it would be to burn gas in a 100% efficient on-site furnace. Transmission losses and all.
A lot of the older heat pump designs will simply cut out at 10-20F and send you to using emergency heat (electric resistive heating which is what you already have). Why would I think you should worry about that?
Why would a highly energy efficient heat pump be illegal in this region?
I'm sure one wouldn't be. On the other hand, your unit barely meets the absolute bare minimum floor standards in 46 states.
Look at the sticker. There's a scale of "least to most efficient" and it shows where your unit lands. If you didn't know, that's the bad end of the scale. 13.4 being the worst units going to northern states, and 19 being - well, the scale is handicapped. Many units are available with SEER2 well into the 30's. But that would make domestic manufacturers look bad.
But again SEER buys you nothing, you need COP or HSPF.
I went shopping for A/C units in 2021 in rural country. Everyone wanted to sell me el-cheapo 13 SEER air conditioning units. Why are they all 13 SEER? Because that was the absolute bottom shelf least efficient unit that could possibly be sold in the northern states. And even those manufacturers had to be dragged kicking and screaming up from 10 SEER, the standard in the 90s.
Meanwhile in Asia.... A/C in the 30s of SEER are common, and they're all reversible heat pumps, coz that's how they roll over there. And they work fantastically - see this video about analysis of their use in Chicago winters, for Pete's sake.
The US industry does not want to make efficient units, because that would cost a tiny bit more at scale.
So more likely than not, you are looking at a tuned-up 1970s heat pump design that was shimmed just enough to be legal in 46 states. *The problem for you is that moldy old tech is laughably terrible at low temperature heat-mode operation...
... which is Your Entire Point Of Buying It.
And again you are after COP @ temperature, not SEER. Heating is a different job than cooling and requires different optimization.
Should I be able to infer that the newer models will be banned soon too, Energy Star just takes time to create the Energyguides?
Yes, units which are deliberately built to barely comply today will not comply in the future. That should be obvious.