Unable to install VST plugins in Sonoma due to security constraints

I am trying to install plugins (vst3, vst, AU etc.) for use in my music software. The new ones get rejected by the OS security as not being made by an Apple recognised developer. This often happened in the past, but pressing the "install anyway" button was able to override that. Now, however, telling it to "open anyway" (the message changed slightly) does not seem to be having any effect.

Any suggestions on how I can defeat this massively intrusive behaviour?

[Re-Titled by Moderator]