That happened to me and my sites were down for four months. I lost 15 years of seo, and $30/mo revenue... They just shut down the Python app engine sites with no notice. I'll never use google cloud.
I wish it were. It was an enormous amount of effort to get my pagerank that high. I suppose I could post my google analytics before and after, but that's not really data I share with the public.
I'm not doubting your site was "down for four months". I'm doubting the part where you said "They just shut down the Python app engine sites with no notice".
Most notably, I know many people who run these types of sites and outside of GAE being mediocre, I've never heard them complain about anything like that.
This was the very first version of app engine that rolled out in 2008 or so. It showed some type of "incompatible version" notice in the admin ui when I noticed it, and when I tried to redeploy the sites using the command line deploy tool. I switched everything to s3.