Many Microsoft Flight Simulator 2020 add-ons that have not yet been tested and officially released for MSFS 2024 may already work.
Here's how you can install them in the new simulator and test them for yourself, including a trick for Orbx Central's client, which would usually require you to have both simulators installed, but the requirement can be bypassed.
Popular third-party developer PMDG released today its rendition of the Boeing 777-200ER for Microsoft Flight Simulator 2024 and MSFS 2020.
Blackbird Simulations released the alpha version of its highly anticipated C-130 Hercules for Microsoft Flight Simulator 2024.
Seattle Tacoma International Airport (KSEA) has been released by MK Studios for Microsoft Flight Simulator 2024 and MSFS 2020.