We can also look at plugins that are AU only, etc, but I assume most of the general public is more interested in VST3, which most everyone could use if supported by Reason (depending on OS support by individual developers).
And this as I understand can even vary between operating systems. For example, something might be VST3 only on macOS, but available in VST2 on Windows (or vice versa).
Is there a good resource out there that already keeps track of this?
Otherwise, we can start keeping track here to rally for support for these formats.
I'll start it off with what I have come across! And maybe I can edit this first post with updates whenever I have time. Let me know of any corrections

Antares
https://www.antarestech.com/
VST3, AU, etc, but no support for VST2
Line6 PodFarm 2.5 (for Mac)
https://line6.com/pod-farm/
macOS doesn't have 64bit VST 2.4 version. Only AU is 64 Bit and VST is 32 Bit.
Roland Cloud Legendary Series
https://rolandcloud.com/catalog/legendary
VST2 available for Windows only, macOS is VST3/AU only
Scuffham S-GEAR (for Mac)
https://www.scuffhamamps.com/download/s-gear-v2
VST3, AU and AAX only
Sinevibes
https://www.sinevibes.com/plugins/
AU only (macOS only)
sonicprojects - OP-X PRO-II (for Mac)
http://www.sonicprojects.ch/opxpro2/macversion.html
AU/VST3 only for macOS
Sonoma Wire Works - DrumCore 4
https://www.sonomawireworks.com/drumcore/
VST3/AU/AAX
Steinberg - Absolute VST Collection
https://new.steinberg.net/absolute/
VST3/AU/AAX
Yamaha
VST editors for their hardware synths are VST3 only.