Hi Kevin,
MySQL isn't deeply incompatible, it's just the out of the box defaults have changed - so it behaves differently from expected.. even down to how it sorts. MariaDB still behaves. If you were very persistent you can probably get MySQL to behave.. but simpler for the project to standardise just on MariaDB
If you have to stick on MySQL you can probably get to v0.9.6 but I'd recommend reviewing subsequent XSS changes and backporting them - htmlspecialchars on public-facing form fields as a minimum. Also check security on uploads/attachment folders;
https://documentation.opencats.org/tech ... -.htaccess
All-in-all, I'd go to 0.9.7.2 with MariaDB and php 7.2.
Russ.