Hi there. In my default installation for a lot of clients I have php7, custom clean ISPConfig theme, DKIM support added and lets encrypt for SSL certs. Now, my question is if anyone have similar setup or you guys from HTF have tested upgrade to 3.1. from this/similar setup? Everything goes fine or there are issues? Custom php7 probably isn't a problem, as you installed it in come custom path, but what if new ispconfig 3.1. is using the same path ? For ispconfig clean theme there shouldn't be issues, because this version doesn't break default theme. DKIM is specific, for present one there could be problems as you also modify a lot of stuff, even in the dbispconfig/ispconfig. For present lets encrypt there could be issues as well, especially if some vhosts are already managed through it. Most of the servers are dedicated/VM without possibility to clone them - so I don't really have option to test this easily, but just live test, which could lead to some downtime.
DKIM is working and the additional changes to dns are intended. You can explain your disbeliefs but i'm pretty sure, that there is nothing wrong with DKIM. The code is used for over 2 years on several servers.
I'm using DKIM patch since it was introduced, so I know that stuff is working perfectly, with all public/private keys added, stuff in email headers and when you're creating mail domain, there's option to add dkim record in DNS zone. So, I didn't say there is something wrong, I'm just asking if someone noticed any problems when upgrading present installation with DKIM
Additional PHP-Versions will work, too. You custom theme won't work anymore as ISPConfig 3.1 has a new UI. Better switcht to default-theme before upgrading to 3.1