I think it's time to point out again that there are some manga sites on ZeroNet that *cannot* be shut down *at all*, thanks to decentralized hosting and Tor. You will need to install ZeroNet and Tor and got to 1+Gallery (zero://1PGaLLNiadMzrmw6MMMnpZnskwFcdgZoS4) then add compatible hubs with the type of content you're interested in. So it's a bit of a hassle to get into, but once you're in you can download for offline browsing, also no ads.
It's mostly H (R-18) content that's available there (10,000 mangas last time I checked)
I still keep using Mangadex because it's awesome and has good quality content, but ZeroNet is my backup plan if it ever goes legs up.