Available as an assembled Module and as a DIY project.
This Module is currently available.
Negative Slew Generator
The ES19 is one of the unique multi-functional patch-programmable modules in the Euro-Serge system. The slew is active in the negative direction only and can be patched to perform a number of synthesizer functions. With the Pulse output applied to the input, the module will regenerate for use as a voltage controlled sawtooth oscillator or pulse generator.
An audio signal applied to the input will be envelope detected, and the complex envelope will be available at the output. If a pulse is applied to the input, the unit will function as an envelope generator with a fast rise time and a voltage controlled fall time.
https://www.elby-designs.com/webtek/euro-serge/es19-negative-slew/es19.htm
These merchants probably sell this module. Huh?