This is precisely what Cloudflare is doing to end users - causing problems like OP (and myriad others) experience by slowing down and/or blocking major chunks of the internet
I understand that it may be viewed as unpleasant, but ultimately if you install a proxy on your end that the server does not like (say an ad-blocker), I don't think it would be fair for the server to say its suffering a MITM attack. Likewise, even if the client is not happy with the third party the server is requesting, it still doesn't make sense to call it a MITM, IMO.