Teoalida

I did NOT solved the problem directly but I upgraded to VPS thus the problem is no longer causing downtime… and I forgot to reply there to help other users who have similar problems.

First, I have told to my hosting company to reset my account. Resource usage was 0, but as soon I installed a blank WordPress, BEFORE installing any plugin or importing any content, the Entry Processes skyrocket, hit the limit and I was not able to upload any content, then after uploading images via FTP, the Entry Processes went down.

By this way I figured out what is the CAUSE of problem:
Entry Processes were rising since October 2014, which was the date when I started a long process of optimizing my website URL structure, reuploading images in custom-named folders and deleted them from /wp-content/uploads also renamed many images. Many images were already uploaded in custom-made folders since initial website creation 2 years ago, but I also optimized them and changed image file names.
I was deleting all images that are no longer in use on MY website, without being aware that during last 2 years other websites may have hotlinked my images and deleting them from my server may have negative impact for me too. Now every time someone access the websites that hotlinked my images, they send a request to my server, and seems that WordPress use more resources when trying to find non-existing images than when the images were existing.
As stated in the first post if this topic, I was already aware that having broken links to images in my server increase resource usage, but I was not aware that images were so much hotlinked and they get broken if I delete images from server.
Physical and Virtual Memory were also rising since March 2015, but they went down once my hosting company reset my account and I reinstalled WordPress, I did not figured out what was the cause of problem.

Theoretical (but impossible) solution: upload back every deleted image, or upload 1×1 pixel images on every URL that was hotlinked. But is impossible to remember the old URLs of hundreds of deleted images that I uploaded last 2 years. I deleted all website backups except the last one. Is there any way to monitor the URL requests?

Indirect way that I solved the problem: I have upgraded to VPS, paying 3 times more money than on shared server. Obviously the deleted hotlinked images still have some impact on resource usage, but since VPS do not have a limit of Entry Processes, “508 Resource Limit Is Reached” never appear, website is just slowed down.
In the future I will be more careful and I will NOT delete images if I am no longer using them.

Now I wonder how deal with this problem the people who buy domain names that hosted a website in the past to build their own website, the URLs of the deleted website may increase resource usage like in my case.

I remember that the ONLY plugin where I saw a different behavior of resource usage was Broken Link Checker, but the spike lasted only as long the checking was done. I fixed links and deleted the plugin anyway.
I have NOT done anything around 15 October or 1 March when entry processes and memory started to rise slowly to infinite.
SSH is available on VPS only. On my shared host there is not possible to see which processes are running.