David, I haven't seen any log with such URL issue, but I'll keep an eye for that.

I don't think it's the app pool being recycled because I don't see any event for application restart nor a memory usage issue in Azure. The consumption stays around 70-82% of the available memory, no noticeable change during CPU spikes. 

I would first suggest following the three troubleshooting paths below: 

1. Analyze the traffic to see if you're getting hit by a person/bot that is overloading the environment. 
   1. This will include reviewing the site logs in all areas.  
   2. If you're able, put a WAF in front of the site and block everything that shouldn't access the site.  For example, we even block all countries for any site that isn't targeting those countries.  
2. Review the environmental resources to see if they need to be increased.  Mariette suggested this earlier, and I tend to trust her instincts in these matters at all times. 😎 
3. Uninstall any/all extensions in DNN that are not actively being used.  The list you posted includes a large number of extensions (in my opinion).  Most sites don't need that many.  

Regarding the logs, you haven't posted anything yet that seems to be related, but you're looking for fatal errors of any kind.  

https://raw.githubusercon.../master/spammers.txt

You can try adding a rewrite rule to restrict these known spammers.