with the advent of the new UI for site contents in SharePoint, the Site Provisioning application in the PnP-Partner-Pack now has some defunct code, specifically the code that was there to hijack the "new subsite" link, as this has radically changed.
You are right. At the time of writing there is no solution or workaround to inject/embed custom links in the Site Contents page. Nevertheless, we are working on it, in order to find a suitable and solid solution. In the meantime, if you need to use the PnP Partner Pack, we suggest to keep the classic UI instead of the modern UI.
Hi Paolo, just looked at the PartnerPack September release and there we still have the same code, is there no development on a fix for this yet?
I don't believe that we can keep force the user community to keep the classic UI for site contents, I haven't seen a setting for this, the only way to do it is for the user to click the "return to Classic SharePoint" link. To be honest they aren't going to do that and in the meantime they can create subsites that are now outside of our governance model.