Is there a place where I can find known exploits relating to specific Froxlor versions? I have found some on https://www.exploit-db.com/ - but I think there are continuous fixes for new discovered. I have to stay on a specific Froxlor version because newer ones are not compatible with my running DKIM implementation... So a way to use this feature is to fix exploits manually by myself.

as they are reported in different ways there is no single page which includes all the things patched over the last 10 years.

Also, if you have a specific code-adjustment that won't allow you to merge the current stable release where issue are actively being fixed, then you should rethink. If you really want/need to do this manually everytime, just follow the commits on https://github.com/Froxlor/Froxlor/commits/0.10.x (I assume you're on 0.10.x) and pick the stuff you want.

OK! So you will supply security fixes on 0.10.x branch in the future? That sounds great!

Not for very long as there is will be newer releases as always. Maybe better migrate your adjustments or contribute them for all if applicable