Well, there is some good news/bad news on the use of Reason 11 in DP 10.11 under macOS 10.15.6/Catalina for me.
DP 10.11 does correctly scan the VST3 plugins - the Reason Rack Plugin and Rack Plugin Effect.
However, DP 10.11 does not see the Reason Rack Plugin VST3 as an instrument. It treats it like an effect and will only load as an effect-- not useful if you want their synths.
The good news is that the AU version of the Reason Rack Plugin does work as an instrument. However, it does not see the AU version of the effect.
So, to use Reason 11 as a plugin with DP 10.11 you will load the
AU to use as an instrument and the
VST3 to use as an effect.
FYI, under macOS 10.15.6, Logic Pro X correctly sees the Reason Rack AU as an instrument and as an effect. In addition, Roland Zenbeats correctly sees the Reason Rack VST3 as an instrument and an effect.
Will be filing a TechLink with MOTU and will let you all know what they say.