Is the transition altitude around Denali still 18 kft AMSL, and, if so, how is this handled?
The US AIP (ENR 1.7 Altimeter Setting Procedures) is pretty clear about the question being asked, and provides an example.

Minimum altitude comes first.

The lowest usable FL is raised, and not just around Denali (Mount McKinley) in Alaska where the ground juts above 18,000 feet, i.e., the lowest FL can be higher than FL 180 in the contiguous United States.



Let's say the area is not marked yellow and is therefore not congested, and no natural reserves or special conditions exist. In this case the minimum safe altitude is 500 feet AGL.

Let's say ground is 20,300 feet as you present, it means the minimum safe altitude is now 20,800 feet (based on local altimeter setting and accounting for cold temperatures). This is what the AIP says in all caps:


  1.2 EXTREME CAUTION SHOULD BE EXERCISED WHEN FLYING IN PROXIMITY TO OBSTRUCTIONS OR TERRAIN IN LOW TEMPERATURES AND PRESSURES.


Now that we've established 20,800 plus correction for temperature (assume none for the sake of explanation), the lowest FL now depends on that 20,800 plus a correction based on pressure via table ENR 1.7-2 (uploaded as image).


  2.1.3 (...) the lowest usable flight level must be the flight level equivalent of the minimum altitude plus the number of feet specified in TBL ENR 1.7-2.


(Bold emphasis mine.)

If it's a rather low-pressure day, then the correction can be upwards of 2,500 feet. Assume that for emphasis. Our total is now 23,300 feet, which makes the lowest usable FL, FL 235 (or as directed by ATC as stated in 91.159 for VFR, which the AIP mentions).

The example the AIP gives:


  The minimum safe altitude of a route is 19,000 feet MSL and the altimeter setting is reported between 29.92 and 29.42 inches of mercury. The lowest usable flight level will be 195, which is the flight level equivalent of 19,500 feet MSL (minimum altitude plus 500 feet).




This answer is based on the USA AIP at the time of writing this, it is generic, and it is not about a specific location, so do not use for flight.