In what version RX7 used to work before? (I see now “They worked fine in the 3.0 version.”) And is it the 32-bit version of RX? Beware that Audacity ships 64-bit Windows binaries by default now (since 3.0.3 actually) and 64-bit hosts apps won’t run 32-bit VST plugins without a bridge. I can run my own Cabbage-written VST2 64-bit plugins just fine in 3.1.3. See “proof” here for instance https://imgur.com/a/wbY4g0e
If you don’t have dumpbin to check the plugin bitness (it only comes with in Visual Studio or the Windows SDK) you can try the more primitive method describe here: How to check if a binary is 32 or 64 bit on Windows? - Super User
It’s not a problem limited to Audacity, by the way
The VST Bridge is a software interface in Cubase 4 to Cubase 8.5 resp. Nuendo 4 to Nuendo 7. It comes into play when you need to […] run a 32-Bit plug-in in a 64-Bit version of Cubase/Nuendo.