Sure I know about fewexamples.
This is how it all started for currently best Bitcoin mixer in the world Chipmixer, they first asked for alpha testers in 2017 and they still have one of the best campaigns in forum:
https://bitcointalk.org/index.php?topic=1923170.0

Mixtum had review campaign in 2019, and they are available in 2022:
https://bitcointalk.org/index.php?topic=5140688.0

In 2020 there was testing campaign for MyCryptoMixer, but I think they are not available anymore on that address.
https://bitcointalk.org/index.php?topic=5235467.0

Latest mixer review campaign was done by Unijoin few months ago in 2022:
https://bitcointalk.org/index.php?topic=5391409.0